A favicon (/ˈfæv.ɪˌkɒn/; short for favorite icon), also known as a shortcut icon, website icon, tab icon, URL icon, or bookmark icon, is a file containing one or more small icons associated with a particular website or web page. A web designer can create such an icon and upload it to a website (or web page) by several means, and graphical web browsers will then make use of it. Browsers that provide favicon support typically display a page's favicon in the browser's address bar (sometimes in the history as well) and next to the page's name in a list of bookmarks. Browsers that support a tabbed document interface typically show a page's favicon next to the page's title on the tab, and site-specific browsers use the favicon as a desktop icon.

In March 1999, Microsoft released Internet Explorer 5, which supported favicons for the first time. Originally, the favicon was a file called favicon.ico placed in the root directory of a website. It was used in Internet Explorer's favorites (bookmarks) and next to the URL in the address bar if the page was bookmarked. A side effect was that the number of visitors who had bookmarked the page could be estimated by the requests of the favicon. This side effect no longer works, as all modern browsers load the favicon file to display in their web address bar, regardless of whether the site is bookmarked.

In 2003, the .ico format was registered by a third party with the Internet Assigned Numbers Authority (IANA) under the MIME type image/vnd.microsoft.icon. However, when using the .ico format to display as images (that is, not as favicon), Internet Explorer cannot display files served with this standardized MIME type. A workaround for Internet Explorer is to associate .ico with the non-standard image/x-icon MIME type in web servers.

RFC 5988 established an IANA link relation registry, and rel="icon" was registered in 2010 based on the HTML5 specification. The popular <link rel="shortcut icon" type="image/png" href="image/favicon.png"> theoretically identifies two relations, shortcut and icon, but shortcut is not registered and is redundant. In 2011 the HTML living standard specified that for historical reasons shortcut is allowed immediately before icon; however, shortcut does not have a meaning in this context.

Internet Explorer 5–10 supports only the ICO file format. Netscape 7 and Internet Explorer versions 5 and 6 display the favicon only when the page is bookmarked, and not simply when the pages are visited as in later browsers.

The following tables illustrate support of various features with major web browsers. Unless noted, the version numbers indicate the starting version number of a supported feature.

The following table illustrates the image file format support for the favicon.

Additionally, such icon files can be 16×16, 32×32, 48×48, or 64×64 pixels in size, and 8-bit, 24-bit, or 32-bit in color depth. The ICO file format article explains the details for icons with more than 256 colors on various Microsoft Windows platforms.