Source : Free On-Line Dictionary of Computing

topic thread
     
        [{Usenet}, GEnie, CompuServe] 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.