Source : Free On-Line Dictionary of Computing

referer
     
         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)