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

topic thread

    (From Usenet, GEnie, CompuServe) A more or
   less continuous chain of postings on a single subject, sent to
   a forum such as a Usenet newsgroup.  To "follow a thread"
   is to read a series of postings sharing a common subject.

   On Usenet these are connected by "Reference" headers.  The
   better newsreaders can present news in thread order
   automatically.

   (2008-02-06)