[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. Not to be confused with the techspeak sense of ‘thread’, e.g. a lightweight process.

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!