topic drift

topic drift n.  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!"