Referer

A referer (sic) or referring page is the webpage that brought the user or referred them to the current page. The referer is part of the HTTP request sent to a server.

Some Web sites log referers, for advertising or other purposes: the referring Web site may receive a small payment for people who follow an ad ("clickthroughs"). Some browsers and security software allow their users to block this information, to protect their privacy.

Reference

• W3C: RFC 2616: Hypertext Transfer Protocol -- HTTP/1.1