In the context of mailing list

Mailing list

A mailing list is a collection of names and addresses used by an individual or an organization to send material to multiple recipients. The term is often extended to include the people subscribed to such a list, so the group of subscribers is referred to as "the mailing list", or simply "the...

s, discussion groups, discussion forums

Internet forum

An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages. They differ from chat rooms in that messages are at least temporarily archived...

, bulletin board

Bulletin board

A bulletin board is a surface intended for the posting of public messages, for example, to advertise things to buy or sell, announce events, or provide information...

s, newsgroup

Newsgroup

A usenet newsgroup is a repository usually within the Usenet system, for messages posted from many users in different locations. The term may be confusing to some, because it is usually a discussion group. Newsgroups are technically distinct from, but functionally similar to, discussion forums on...

s, and wiki

Wiki

A wiki is a website that allows the creation and editing of any number of interlinked web pages via a web browser using a simplified markup language or a WYSIWYG text editor. Wikis are typically powered by wiki software and are often used collaboratively by multiple users. Examples include...

s a contribution is off-topic if it is not within the bounds of the current discussion, and on-topic if it is.

Even on very specialized forums and lists off topic posting is not necessarily frowned upon, but a common netiquette

Netiquette

Netiquette is a set of social conventions that facilitate interaction over networks, ranging from Usenet and mailing lists to blogs and forums. These rules were described in IETF RFC 1855. However, like many Internet phenomena, the concept and its application remain in a state of flux, and vary...

 convention is to mark a new off-topic posting or email by beginning it with "OT" - for example in a forum discussing the Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 operating system someone might post: "OT: Wow, did you feel that earthquake?".

Posting off-topic messages to deliberately aggravate other members is a form of trolling.