When visiting a webpage, the referer or referring page is the URL of the previous webpage from which a link was followed.

More generally, a referer is the URL of a previous item which led to this request. The referer for an image, for example, is generally the HTML page on which it is to be displayed. The referer field is an optional part of the HTTP request sent by the browser program to the web server.

Many web sites log referers as part of their attempt to track their users. Most web log analysis software can process this information. As referer information can violate privacy, some browsers allow the user to disable the sending of referer information. Some proxy and firewall software will also filter out referer information, to avoid leaking the location of non-public websites. This can in turn cause problems: some servers block parts of their site to browsers that don't send the right referer information, in an attempt to prevent deep linking or unauthorised use of images (bandwidth theft).