1. Of, relating to, or being something that follows up
2. Done, conducted, or administered in the course of following up persons

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.

A second (or subsequent) examination; SYN. reexamination, review.
A follow-up, follow-up call, follow-up visit, and so on, is a return to something that was important to you previously because you now have more information or more time or because you want to make sure some effort you made previously is correct or effective.

1. To pursue.
2. To continue a previous discussion, esp. to see if actions have been performed.