a piece of work that exploits or builds on earlier work; followup
his new software is a follow-up to the programs they started with
an activity that continues something that has already begun or that repeats something that has already been done; followup
a subsequent examination of a patient for the purpose of monitoring earlier treatment; followup, reexamination, review
Microsoft Computer Dictionary
n. A post to a newsgroup that replies to an article. The follow-up has the same subject line as the original article, with the prefix “Re:” attached. An article and all of its follow-ups, in the order they were received, constitute a thread, which a user can read together using a newsreader.