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)