referer

from The Free On-line Dictionary of Computing (8 July 2008)

referer
referrer

   <World-Wide Web> A misspelling of "referrer" which somehow
   made it into the {HTTP} standard.  A given {web page}'s
   referer (sic) is the {URL} of whatever web page contains the
   link that the user followed to the current page.  Most
   browsers pass this information as part of a request.

   (1998-10-19)