Source : Free On-Line Dictionary of Computing

topic drift
     
         Term used on GEnie, {Usenet} and other electronic
        fora to describe the tendency of a {thread} to drift away from
        the original subject of discussion (and thus, from the Subject
        header of the originating message), or the results of that
        tendency.
     
        Often used in gentle reminders that the discussion has strayed
        off any useful track.  "I think we started with a question
        about Niven's last book, but we've ended up discussing the
        sexual habits of the common marmoset.  Now *that's* topic
        drift!"
     
        [{Jargon File}]
     
        (1996-05-29)