Something is
on-topic if it is within the bounds of the current discussion. This term is normally used in the context of
mailing lists, discussion groups,
bulletin boards[?] and
newsgroups, all of which are generally established for the discussion of specific topics. On-topic postings are not generally required to be indicated, but in some groups on-topic discussion is so rare that the header is marked accordingly.
The opposite of on-topic is off topic.