HTTP Referer
A header defined by HTTP that gives a server the URL of the page containing the link/embed/img that has caused the resource in question to load.
Originally a misspelling (in English, something that "refers" would properly be spelled with a double r, as in "referrer"), the spelling (with a single r) has become the established standard (in a sense become a new word/concept itself).
HTTP referers can be used by web statistics scripts to track how pages on a site are linked to from outside sites.