On-topic: Meaning (information, definition, explanation, facts)

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.