More generally, a referrer is the URL of a previous item which led to this request. When visiting a web page, the referrer or referring page is the URL of the previous web page from which a link was followed. 'Referer' has since become a widely used spelling in the industry when discussing HTTP referrers usage of the misspelling is not universal, though, as the correct spelling 'referrer' is used in some web specifications such as the Referrer-Policy HTTP header or the Document Object Model. The misspelling was set in stone by the time (May 1996) of its incorporation into the Request for Comments standards document RFC 1945 (which 'reflects common usage of the protocol referred to as 'HTTP/1.0'' at that time) document co-author Roy Fielding remarked in March 1995 that 'neither one (referer or referrer) is understood by' the standard Unix spell checker of the period. The misspelling of referrer was introduced in the original proposal by computer scientist Phillip Hallam-Baker to incorporate the 'Referer' header field into the HTTP specification. As of March 2021, by default Chrome, Chromium-based Edge, Firefox, Safari default to sending only the origin in cross-origin requests, stripping out everything but the domain name. To mitigate security risks, browsers have been steadily reducing the amount of information sent in Referer.

This entails a loss of privacy for the user and may introduce a security risk.

Web sites and web servers log the content of the received Referer field to identify the web page from which the user followed a link, for promotional or statistical purposes. In the most common situation, this means that when a user clicks a hyperlink in a web browser, causing the browser to send a request to the server holding the destination web page, the request may include the Referer field, which indicates the last page the user was on (the one where they clicked the link). By checking the referrer, the server providing the new web page can see where the request originated. In HTTP, ' Referer' (a misspelling of Referrer ) is the name of an optional HTTP header field that identifies the address of the web page (i.e., the URI or IRI), from which the resource has been requested.

Wireshark filter http header