thread

thread n.  [Usenet, GEnie, CompuServe] Common abbreviation
   of `topic thread', a more or less continuous chain of postings on
   a single topic.  To `follow a thread' is to read a series of
   Usenet postings sharing a common subject or (more correctly) which
   are connected by Reference headers.  The better newsreaders can
   present news in thread order automatically.

   Interestingly, this is far from a neologism.  The OED says:
   "That which connects the successive points in anything, esp. a
   narrative, train of thought, or the like; the sequence of events
   or ideas continuing throughout the whole course of anything;"
   Citations are given going back to 1642!