Octiv Digital
Display & Remarketing Ads Management

Digital Marketing Glossary

What is an alt tag? What is a website crawl? What is display advertising?

Welcome to Our Digital Marketing Glossary!

There are a lot of terms and phrases in the world of online marketing, and we want to make sure you understand the jargon as best as possible. Browse through the alphabetized list of terms and let us know if you have any questions.



200 Response: A header response letting users and search engines know the content is found. Your customer-facing pages should serve a 200 responses.

301 Redirect: A permanent redirect to let search engines and users know the page has moved. This type of redirect is used anytime you create a new page and migrate traffic to it from an old URL.

302 Redirect: A temporary redirect to let search engines and users know the page has moved, but it will be coming back. This redirect is used best for promotional/seasonal URLs, temporary page removals, and site migrations.

404 Response: A header response letting search engines and users know the content of a page is gone or missing. Broken or incorrectly used internal links will result in a 404 response. Search engines will try to crawl 404 URLs a few times, allowing you to fix the issue with the URL.

410 Response: Similar to a 404 Response, but indicates the content of the page is permanently gone. Search engines will no longer try to crawl the URL.

500 Response: A 500 error indicates an issue with the server. Contact your hosting provider if your site is showing 500 error responses.


Adobe Analytics: A analytics platform as part of the Adobe Marketing Cloud. Similar to Google Analytics, but offers users additional segmentation and analysis tools.

Ahref: The HTML attribute used to link from one page to another.

Ahrefs.com:backlink analysis tool used for search engine optimization. The tool allows its users to validate where links are coming from and assess their quality.

Algorithm: The mathematical functions, processes, and logic machines use to return results in search engines.

Algorithm Update: When a search engine makes an update to its algorithm, it changes the way results are returned. Algorithm updates result in positive or negative ranking changes, depending on the criteria of the update.

Alt Text: A tag used in conjunction with images to let crawlers know what an image is about. It’s highly recommended to use keywords for alt text to help search engines understand what the image is.

Analytics: The measurement of digital traffic. Most websites use Adobe Analytics or Google Analytics to track performance.

Anchor Text: The text placed within the link attribute.

Attribution: Attribution is a term related to the performance of marketing channels in a visitor’s path. Most advertising platforms use a last-click attribution model, while some use a first-click, position-based or linear. Attribution models will differ depending on a business’s goals and the customer’s typical time to purchase.

Authority: Authoritative sites are those that typically rank well and have a lot of credibility. For example, an authoritative site in the home electronics market is Best Buy.

Average Order Value (AOV): AOV is the average basket total for each order an e-commerce store receives. High average order values indicate people are finding more products through cross-selling or up-selling while low average basket totals reflect an opportunity to improve on-site marketing.


Backlinks: A link pointing to your site from another site. Backlinks are used as part of a search engine’s algorithm to give credit to your site. Backlinks can either be nofollow or dofollow depending on the context of the link.

Banner Ads: A banner is a type of ad unit used in display and retargeting campaigns. Banners can be static images, GIFs or responsive creative built through Google Ads.

Bing: A search engine from Microsoft and typically the alternative to the Google search engine.

Bing Ads: Microsoft’s ad network serves search and shopping ads on the Bing search engine.

Black Hat SEO: A type of search engine optimization in which none of Google’s compliance and quality standards are adhered to. The SEO intentionally uses risky tactics to rank sites knowing they could be discovered and removed by Google completely.

Blog: Short for “weblog”, this is a section of a website that provides company updates, industry-relevant posts, and customer-centric information. Often used to provide freshness on a website.

Bounce Rate: An analytics metric expressed as a percentage to measure how often a user reaches a landing page and immediately clicks the back button. If a site correctly answers a user’s query and provides a good experience, expect the bounce rate to be low.

Browser: The internet is accessed through a web browser like Safari, Chrome, Firefox, or Internet Explorer. When a browser requests a URL, a web server returns the URL’s content.

Bumper Ads: A type of YouTube ad that plays before any video content.


Canonical Tag: A canonical tag is a link element to inform search engines of a primary page in a set of similar pages. Primarily used in enterprise SEO, the canonical helps Google to know which page of a variety of similar pages is the one to rank. For example, a user may be able to navigate to a Vans shoes URL with blue selected as the color through the URL example.com/shoes/vans/blue, but they can also reach a very similar URL like example.com/shoes/vans/blue/size-10. Because the content does not change substantially, a canonical needs to be set for the preferred URL.

Cascading Style Sheets (CSS): Used in website design and development, CSS is the stylesheet that is referred to for fonts, colors and any element used to style a page from basic HTML to customer-facing content.

Citation: Used for local SEO campaigns, a citation is a listing on the internet of a business’s name, address, and phone. Well-optimized citations help improve performance in local listings.

Clicks: In Google Ads and in SEO, a click is measured anytime a user clicks on an ad or organic listing from the Google search engine result page.

Click-Through-Rate (CTR): Expressed as a percentage, CTR measures how often an ad or organic listing in a search engine is clicked. For example, a URL with 1000 impressions and 10 clicks would have a 1% CTR.

Content: Content is any kind of media that is used online. From text blocks to images, videos to infographics and everything in between, content is the bread and butter of the internet.

Content Management System (CMS): The backend of a website where all content pages are created or updated. Typical CMS platforms include WordPress, Salesforce Commerce Cloud (for e-commerce), Magento and Squarespace.

Conversion: As defined by the advertiser, a conversion is any user action with a website or marketing campaign. It could mean an order for an e-commerce store, a sign-up for a lead generation site or a phone call if that is the campaign priority.

Conversion Funnel: A conversion funnel is referenced when gauging where a customer is in their journey to buying online. Customers at the top of the conversion funnel are in a research phase, while those who are mid-funnel have evaluated options and are close to purchasing, and those at the bottom of the funnel are ready to convert.

Conversion Rate (CVR): Expressed as a percentage, the measurement of how many conversions happen in relation to traffic. For example, if a website converts 10 orders from 1000 visits, the site converts at 1%.

Conversion Rate Optimization (CRO): The process of evaluating conversion rate issues as related to user experience, and making improvements to increase the conversion rate.

Cookie: A tiny piece of data collected by a website and is stored on the visitor’s device. Collecting cookies helps websites remember visitor’s information to quickly retrieve items left in a shopping cart, information on forms or to load pages a user has already visited. Most websites offer a cookie policy to let users know their information is being collected and stored for their benefit.

Cost-Per-Acquisition (CPA): A metric of paid advertising that measures the overall advertising cost of acquiring a new customer. Lead generation campaigns typically use a CPA as the primary success metric.

Cost-Per-Click (CPC): Often expressed as “PPC“, a CPC is the cost for every click an ad receives. Some industries require higher CPCs to be competitive while lower CPCs mean less traffic.

Cost-Per-View (CPV): Cost-per-view is the equivalent of a CPC, but it’s used in video advertising. Advertisers must determine how competitive they want a video ad in YouTube to be and set the corresponding CPV bid.

Cost-Per-Thousand-Impressions (CPM): Cost-per-thousand-impressions is similar to a CPC, however it allows advertisers to set a bid for every thousand impressions an ad receives. Most often used for display campaigns.

Crawling: In SEO, crawling occurs when a search engine visits a site, reads through content, follows links to additional pieces of content and then determines what content should be indexed.

Crawl Budget: In SEO, search engines have limited resources to crawl millions of sites per day and crawl budget refers to how much bandwidth is used. Large (typically e-commerce) sites have to consider crawl budget to help guide search engines to the most important pages


Directory Listings: Used for local marketing strategies, directory listings are a way to increase visibility for small businesses. Directory listings have also been used as part of link-building for small businesses.

Display Ads: See banner ads. Display ads are used through the Google Display Network to show image-based ads on a variety of external sites.

Dofollow Link: A type of link used to pass authority from one page to another. If not specified with rel=”nofollow”, links will be considered dofollow.

Domain Name: The web property where a site’s content exists. Domain names are registered through domain name registrars like GoDaddy or Name.com.

Duplicate Content: A phrase used in context with SEO when a page’s content is plagiarized from another page, either intentionally or under misdirection. Receiving a duplicate content penalty from Google means the content is being plagiarized from original sources. For optimal SEO, avoid duplicate content.


E-Commerce: Conducting business online through a storefront. Examples of e-commerce businesses include Amazon, Best Buy or anywhere a product can be purchased.

Email Service Provider (ESP): In email marketing, campaigns are managed through an email service provider like MailChimp, Constant Contact, Silverpop, Exact Target, etc.

Enterprise SEO: The type of search engine optimization strategy used for large-scale (typically e-commerce) businesses.


Facebook: One of the largest social media networks used by businesses and individuals. Facebook advertising allows businesses to layer multiple levels of targeting into a campaign to place content in front of a very direct audience.

Faceted Navigation: Faceted navigation is how a user filters through search results on an e-commerce site to refine product results. Common examples are filtering products by size, color, gender or category.

Featured Snippet: In SEO, a featured snippet may appear at the top of results above traditional organic listings. The results are usually tied to a website’s formatted content and seek to answer a user’s query.

File Transfer Protocol (FTP): File Transfer Protocol (FTP) is how the server side of a website is accessed. FTP applications will open a file system of your site’s files and allow you to edit site documents from a local text editor.


Google: The largest search engine in the world and the reason for search engine optimization/search engine marketing.

Google Ads: The advertising platform used for Google products like search, shopping, video, and display campaigns.

Google Analytics: The marketing analytics platform used by most websites and marketing channels. It provides insight into website behavior, location data, browser data, channel breakdowns, and much more.

Google Business Profile: Google’s product for listing local business information, formerly known as Google My Business. A Google Business Profile is what shows in local listings when it is well-optimized.

Google Display Network (GDN): Used with Google’s Adsense product, the Google Display Network is all of the sites where display advertisements are allowed to be shown. There are currently ~2 million websites active in the network.

Google Merchant Center: Use for product listing ads in Google, the Merchant Center is where data feeds are sent and evaluated for advertising use. The Merchant Center will flag any errors in product data feeds for advertisers to correct before setting up campaigns.

Google My Business: See Google Business Profile.

Google Search Console: Formerly referred to as Google Webmaster Tools, the Google Search Console is the tool used by SEOs to understand how websites are performing from the perspective of Google. It offers helpful reports, shows crawl information, and allows users to better manage the performance of their site.

Google Tag Manager: A platform that allows multiple marketing and analytics tags to be installed seamlessly on websites through the use of one Javascript container.

Gray Hat SEO: A type of search engine optimization in which Google’s compliance and quality standards are not strictly followed. The SEO may perform some tasks in an attempt to game search results.


Hashtag: An element used in social media to tag content. Posting content with popular hashtags can lead to higher engagement rates.

Head Term: In search marketing, a head term refers to a one or two-word user query. For example, “roofing” is a head term while “Phoenix roofer” is a torso term, and “Phoenix, AZ roofing company with the most reviews” is a long-tail term.

Heading Tags: In web content creation and SEO, heading tags help establish the architecture of a page. Header tags include H1, H2, H3, H4, H5 and H6.

Host: A host is the web server where a website’s content is placed. When a user tries to reach a URL, the web browser makes a request to the webserver to retrieve the content. Popular web hosts include GoDaddy, Host Gator, Siteground, and WPEngine (for WordPress sites).

HTACCESS: Often represented as .htaccess, a .htaccess file is a configuration document that lives on a web server using Apache (WordPress). The file may contain redirection directives, caching scripts, and permalink settings.

HTTP / HTTPS: Short for hypertext transfer protocol, HTTP defines how content is formatted on the web and transmitted through browsers and servers. HTTPS, short for hypertext transfer protocol secure, is a secure version of the protocol that ensures the user’s information is protected. Using HTTPS is preferred for SEO as it establishes trust and is a must for e-commerce websites where personal information is shared.

Hyper Text Markup Language (HTML): The language used to tell a browser to display a web page. HTML is used in conjunction with CSS and Javascript to develop rich, user-friendly websites.

Hyperlink: The web format that connects one URL to another URL, either internally or externally. Hyperlinks are the essential building blocks of the internet.


IDX: Short for Internet Data Exchange, IDX is the bank of MLS data accessed by real estate websites. A realtor’s site must subscribe to an IDX provider to display real estate listing info.

Impression: In digital marketing, an impression occurs whenever an ad or organic listing is shown. Even if a user does not click on the ad, the ad content was still displayed and an impression occurred.

Impression Share: In paid search advertising, impression share is the measurement of how many impressions an ad has in relation to other advertisers. For example, if an advertiser showed their ad three times for ever ten user search queries, they would have a 30% impression share. Impression share is a key metric of product listing ad campaigns to understand how competitive product data is.

Indexing: In SEO, indexing occurs when a URL is included in a search engine’s index. Being included in the index is critical for improving ranking, visibility and traffic to a website.

Instagram: One of the largest social media networks used by businesses and individuals. Instagram is owned by Facebook and is primarily driven by images and video content.

Internal Links: Within a site, pages internally link to each other. Proper optimization of internal links and ensuring no link is broken can help with SEO efforts.


Javascript: A type of web language used in web development that works with web browsers to display interactive elements to users. Javascript works in conjunction with HTML and CSS to create improved user experiences on most modern websites.

Joomla: A type of content management system used by small businesses.


Keyword: The marketing language for what users enter into a search engine like Google or Bing. Keywords are used to understand the user’s intent and craft marketing collateral around the keyword or set of key phrases. Keywords are used for SEO and paid search advertising.

Keyword Density: In SEO and content creation, keyword density refers to the number of times a keyword or keyphrase is used in a particular piece of content. Content with really high keyword density is likely to be considered spammy by search engines and may be ignored.

Keyword Research: Keyword research is the process of understanding what users are typing into search engines and identifying trends to capitalize on opportunities. There are several platforms used for keyword research including SEMRush, Ahrefs, Worstream, and the Google Keyword Planner.


Landing Page: In digital marketing, a landing page is any web page created with marketing intent. The page could be used to inform users of a product or service, acquire an email address, drive a form fill or a phone call.

Link Profile: In SEO, a link profile refers to the backlinks pointing to a domain. Link profiles should be clean and comply with Google’s standards to result in ranking increases.

LinkedIn: A type of social network that is popular among business professionals. Business owners can use LinkedIn‘s advertising capabilities to reach customers in a professional network.

Local SEO: The type of SEO employed for small businesses who need to rank in a regional geographic market.

Long-Tail Term: Long-tail refers to a type of user query that contains many details. In search marketing, a long-tail key phrase may be comprised of five words or more. For example, “roofing” is a head term while “Phoenix roofer” is a torso term and “Phoenix, AZ roofing company with the most reviews” is a long-tail term


Magento: A type of content management system that offers e-commerce capabilities. It’s one of the most used content management systems for online stores.

Medium: In analytics reporting, medium refers to the channel that was used to acquire traffic. Examples of mediums include CPC (paid search), organic (SEO), email, direct, and referral (social links, related websites).

Meta Data: In SEO, metadata refers to the elements that a search engine reads to understand and grade a page. Metadata includes title tags (shown in the browser title), meta descriptions (the snippet of text in search engine results), and meta keywords (deprecated by most search engines).

Meta Robots: Meta robots is a tag used to allow or disallow a search engine from indexing a page. If a meta robot has no index tag set for a page, search engines will be directed to exclude the page from the results.

Minification: Minification is a web development technique used to clean up HTML pages and stylesheets of any unnecessary white space. Proper minification helps pages load faster and can have SEO benefits.


Name, Address, Phone (NAP): NAP is short for name, address, and phone information that displays in citations and directory listings on the web. Ensuring this information is accurate is critical to success with local SEO.

Negative SEO: Negative SEO is a form of search engine optimization to disparage competitors. It typically involves pointing spammy links to a competitor’s site so as to hurt their reputation in search engines.

Nofollow Links: In SEO, a nofollow link is any link that is meant to not pass authority. Nofollow links are commonly used for affiliate links, sponsored links, and links embedded in user-generated content. If the rel=”nofollow” attribute is not defined for a link, it is considered a dofollow link.

Noindex: In SEO, a noindex command is used when webmasters wish to have a URL excluded from search results. The noindex action is used in a meta robots tag.


Off-Page SEO: The type of SEO that happens outside of a website. Off-page strategy is inclusive of backlink development, outreach, and relationship building, citation building for local campaigns, and improving social signals.

On-Page SEO: The type of SEO that happens on a website. On-page is inclusive of title tags, descriptions, content optimization, internal link development, page speed analysis, and technical enhancements.

Over-Indexing: In SEO, over-indexing occurs when too many pages are indexed in Google or Bing. In most cases, over-indexation occurs when tracking parameters are crawled or duplicate pages exist.


Pagespeed: Pagespeed refers to the amount of time (typically measured in seconds) that it takes to load a web page. Sites that are able to load pages fast tend to retain visitors and perform better in organic rankings. Pagespeed is measured for both desktop and mobile experiences.

Parameter Handling: Parameter handling is a feature of the Google Search Console that allows webmasters to control how websites are crawled. When a site uses links with parameters for tracking, advertising, or understanding user behavior, it’s best to use parameter handling to limit how many pages search engines crawl.

Pay-Per-Click (PPC): Pay-per-click, also referred to as search engine marketing, is a type of digital advertising in search engines. Advertisers show an ad based on a user’s search query/intent and pay for the placement based on a max cost-per-click.

Penalty: In SEO, a penalty refers to a demerit from Google for practicing some form of illicit behavior in effort to “game” search results. Historical penalties from Google have included duplicate content penalties and link spam penalties. Once penalized, maintaining rankings is nearly impossible.

Performance Marketing: Performance marketing is any type of advertising that ties into a specified budget and return (ROAS) target. Most forms of performance marketing include paid search, product listing ads, retargeting campaigns and email marketing.

Pixel: A snippet of code that tracks user behavior on a website across different devices and browsers. Pixel tracking is used by many advertising platforms including Facebook and Google.

Position: In SEO, position refers to the rank of the organic listing for a website. SEOs may refer to pages as being in position one, position five, position twelve, etc.

Position Zero: In SEO, when Google chooses to show content above organic results, it’s considered position zero. The content shown in this section is also referred to as a featured snippet.

Private Blog Network: A private blog network or PBN is used to try to manipulate search results through backlinks. The links are placed on a privatized group of sites with the sole intention of driving up search engine rankings. Private blog networks are not permitted by Google’s quality standards and often result in a manual penalty.

Product Listing Ad (PLA): Product listing ads are a type of ad unit used in search engines to show product data. PLAs, also referred to as shopping ads, use data from a product feed to create ads consisting of images, titles, prices and review / rating information. PLAs work in conjunction with the Google Merchant Center and Google Ads.

Product Information Management System: A product information management system (PIM) is used by e-commerce businesses to house their products. For large-scale retailers, a PIM is typically a 3rd party component of a site as it requires a lot of space to operate.


Query: A query is what a user enters into a search engine like Google or Bing.

Query Negation: A strategy used in paid advertising (search and shopping campaigns) to appropriately funnel out poor-performing traffic. Negating queries occur at the campaign level or ad group level and can be based on lists of negatives.


Ranking: See position. In SEO, ranking refers to where an organic listing is shown on a search engine results page.

Reconsideration Request: In SEO, a reconsideration request happens when a website has been removed from search engine results in Google. Google may choose to remove (also referred to as de-indexing) a website if it contains spam content or is used to “game” search results in any way. Webmasters must submit a formal reconsideration request to Google to have a website included in search results again. Most reconsideration requests are a result of some type of penalty from Google.

Redirection: When temporarily or permanently removing a URL, redirection is used to direct users to the new URL. Redirects are either 301 (permanent) or 302 (temporary).

Remarketing: Remarketing is a paid search strategy to show ads to customers who have already interacted with a site in some way. Advertisers can establish remarketing lists for search ads (RLFSA) to increase bids for users who have already visited a site and are more likely to convert.

Reputation Management: Reputation management is a form of search engine optimization to suppress negative content about a business, brand or individual in favor of promoting positive information.

Retargeting: Similar to remarketing, retargeting ads are used to regain the interest of users who have already interacted with a site. Retargeting is used in display banner ad format and can follow users around the web, reminding them of a product they showed interest in, a promotion they showed interest in or some other interaction they had with a site. Retargeting campaigns tend to convert really well due to creative used.

Responsive Design: A type of web development where content is made and sized for all devices (desktop, tablet and mobile).

Return On Ad Spend (ROAS): Expressed as a percentage, ROAS is the revenue return on each ad dollar spent. For example, if an advertiser spends $100 in ads and makes $500 in sales from the ads, the ROAS is 500%.

Robots.txt: A robots.txt file is used to inform search engine crawlers if they are allowed to read a website. A common analogy is that a robots.txt file is the doorway to a website – if it’s closed with a disallow sign, search engines will go away. If it’s open for business, a crawler is welcome to read a site and any of its resources.


Salesforce Commerce Cloud (SFCC): An e-commerce content management system from Salesforce. Formerly named Demandware until the company was acquired by Salesforce in 2016.

Search Engine Marketing (SEM): Search engine marketing is the strategy used to pay for placement in Google results. Advertisers set a CPC, create ad content and then bid for positioning in results.

Search Engine Optimization (SEO): SEO is the process of improving ranking and visibility in search engines like Google and Bing. Optimization techniques are comprised of technical analysis, content development and link acquisition.

Search Engine Result Page (SERP): Short for search engine result page, the SERP is the landscape of results displayed in Google or Bing anytime a query is made.

SEMRush: A marketing research platform that is used for keyword research, backlink analysis, competitor analysis and more.

Screaming Frog: A crawling tool used for SEO. The tool crawls websites in a way that mimics Google, then provides data on key on-page SEO elements.

Spam: In SEO, spam refers to anything used to try to game a search engine. For instance, content filled with keywords may be considered spam or acquiring links from questionable locations could be considered spam. In email marketing, spam refers to unwanted messages usually with a marketing intent.

Squarespace: A content management system that is ideal for businesses just starting out who need a basic template to get a site up and running.

Structured Data: In SEO and content creation, structured data (also referred to as schema markup) is used to further instruct search engine crawlers what a page’s content is about. Developers implement structured data based on schema.org guidelines.

SSL Certificate: An SSL Certificate is required to validate a website as secure. It’s used for HTTPS protocol and works to secure logins, data transfers and credit card transactions.


Title Tag: In SEO, a title tag is a metadata element that informs search engine crawlers of the name of a page. The title tag is displayed in search engine results and is often what causes a user to click through to the corresponding page.

Torso Term: In search marketing, a torso or body term refers to a user’s query that consists of a few words. For example, “roofing” is a head term while “Phoenix roofer” is a torso term and “Phoenix, AZ roofing company with the most reviews” is a long-tail term.

Twitter: A social media network that allows its users to tweet micro blogs and use hashtags in posts. Twitter is used by individuals and businesses, and offers advertising capabilities. As of 2023, Twitter was rebranded to X.


Uniform Resource Locator (URL): A URL is an element of the world wide web and identifies the address of a web page. When entered into a web browser, the browser fetches the URL from a web server to display the content of the page.

User Experience (UX): In web design and development, designing with user experience in mind will drive better engagement with the site and retain visitors. UX may include aesthetics, fonts, colors, functionalities on a site, and more.

User Interface (UI): A user interface refers to the backend dashboard of websites or marketing platforms where a user would interact with a website in one way or another. For example, the backend of Google Ads is a UI for advertisers to set up campaigns.


Visits: In analytics and measurement of marketing-related traffic, visits refers to the users clicking through to a site. Unique visits occur when a completely new user is tracked while returning visits are inclusive of people who have already interacted with a site.


Web Design: The customer-facing look and feel of a website. Web design is usually comprised of wireframes and Photoshop documents before actual development begins.

Web Development: Web development refers to the coding required to bring a website to life. Web development includes HTML, CSS, Javascript and any other web language required by a site.

Website: A website is a digital property where multiple documents and media are grouped together, then displayed to users in a visually appealing manner.

White Hat SEO: A type of search engine optimization in which Google’s compliance and quality standards are followed. The SEO does not participate in any questionable tactics to try to game search results in Google.

WordPress: A content management system (CMS) that is ideal for small-to-midsize businesses that require customizations in their website design


XML Sitemaps: In SEO, XML sitemaps are used to inform search engine crawlers about pages and other documents that exist on a site. Extensible markup language (XML) creates a document tree for a crawler to read and comprehend pages of a website. XML sitemaps can be used for page URLs, media URLs (images, video) and other forms of content that need to be indexed in a search engine.


Yahoo: A search engine that provides results based on Bing’s algorithm. In the landscape of search engine market share, Yahoo is often considered third on the list of users.

Yelp: Yelp is a local review site that allows small business to publish their information and attract new customers. Yelp is an essential tool for local marketing campaigns.

YouTube Ads: A type of video advertisement used on YouTube and partner video sites in the Google Ad network. Ads are shown in a few different formats depending on the advertiser’s goal.


Z-Index: In web design and development, z-index refers to the stacking order of elements on a web page along the vertical (z) axis. Determines which elements overlap when they overlap. The z-index property applies to positioned elements (that is, elements with relative, absolute, fixed, or fixed position values) and accepts a numeric value. The higher the number, the higher the stacking order of the item. By default, an element’s z-index is auto. In other words, the stacking order of elements is determined by their position and order of appearance in the HTML document. Using z-index is essential when creating complex layouts such as drop-down menus or modal windows, where the position and visibility of elements need to be carefully controlled.