A contribution is on-topic if it is within the bounds of the current discussion, article, etc. The term is normally used in the context of mailing lists, discussion groups, bulletin boards, newsgroups, and wikis. A mailing list is a collection of names and addresses used by an individual or an organization to send material to multiple recipients. ... Well-used bulletin board on the Infinite Corridor at MIT, November 2004. ... A newsgroup is a repository, usually within the Usenet system, for messages posted from many users at different locations. ... A wiki (pronounced , , or ; see section Pronunciation) is a web application that allows users to add content, as on an Internet forum, but also allows anyone to edit the content. ...

The opposite of on-topic is off-topic. A contribution may be off-topic with regard to a thread/page/etc., or with regard to the whole site/newsgroup/etc., which is often established for dealing with specific topics.

Being on-topic is not generally required to be indicated, but in some groups on-topic discussion is so rare that the header is marked accordingly.