The Jargon File (version 4.4.7, 29 Dec 2003):

topic group
 n.

    Syn. forum.


The Free On-line Dictionary of Computing (30 December 2018):

forum
fora
topic group

    (Plural "fora" or "forums") Any discussion group
   accessible through a dial-in BBS (e.g. GEnie, CI$), a
   mailing list, or a Usenet newsgroup (see network,
   the).  A forum functions much like a bulletin board; users
   submit postings for all to read and discussion ensues.

   Contrast real-time chat or point-to-point personal e-mail.

   [Jargon File]

   (1998-01-18)