OffTopic

Off-topic content is content that does not further the wiki's mission. Most wikis' rules about what is on- or off-topic are implied by the about page, which describes the wiki's purpose. There may also be an explicit "inclusion policy", "deletion policy", or "what this wiki is not" page stating what content is acceptable and unacceptable. Sometimes there are dissident users who advocate a "This wiki is a community" policy, but they tend not to prevail in completing eliminating the banning of some off-topic content from the wiki. Sometimes people advocate benign neglect of the issue, thinking that off-topic discussions will tend to peter out on their own if left alone for awhile.

Defining
On some wikis, it's more difficult than on others to draw a clear line as to what is off topic. E.g., a conversation about abortion on a wiki devoted to technical documentation would be clearly irrelevant. However, talk could potentially drift off-topic even there, by means of analogies (e.g. "You say that users should be allowed to delete their own user pages, because they created them. By that logic, abortion should be allowed!") How strictly the rules against off-topic discussions are enforced may depend on the forum (e.g. user talk page discussions are more likely to be left alone than discussions on a community forum) and whether the users involved are okay with the discussion going on (e.g. if the user whose talk page it is doesn't want to discuss it, then he may complain about its being off-topic and ask it to end).

Inclusionism and deletionism
Content can be off-topic for the page in question, or for the entire wiki. Inclusionist and deletionist philosophies can come into play here. Some wikis, such as RationalWiki, have debated whether to expand their mission and become general encyclopedias. (In the case of RationalWiki, this idea was rejected, but the inclusion criteria were left so loose as to still potentially encompass much of what a general encyclopedia would include.) Some general encyclopedias, such as Wikipedia, continually debate how notable a subject must be in order to be worthy of inclusion; this was theoretically settled with the wikipedia:Project:Notability, but there is still some room for interpretation of what meets the criteria.

When a wiki has no established policies concerning what is worthy of, and suitable for, inclusion, then those who add such content do so at their own risk, because there could be a purge of all that content as soon as the community decides to impose a new norm. This could also happen if the wiki has a relatively inclusionistic norm, and then the norm changes.

Spam
Some off-topic content, especially by users who have made no other contributions to the community, and are deemed unlikely to, may be regarded as spam. The quintessential example would be linkspam added by automated scripts. There are grey areas, though, such as a user's making a first edit to his userpage to introduce himself and link to some of the websites he is involved in. It could be deemed unfriendly to delete the page and block him; but on the other hand, there is possibility that will be his only edit.

Rules

 * https://meta.wikimedia.org/wiki/Project:About
 * https://meta.wikimedia.org/wiki/Project:Inclusion_policy
 * https://meta.wikimedia.org/wiki/Project:What_Meta_is_not
 * mw:Project:About
 * mw:Project:Deletion
 * wikipedia:Project:Writing_better_articles
 * wikipedia:Project:Talk_page_guidelines
 * wikipedia:Project:User_pages
 * wikipedia:Project:Refactoring talk pages
 * http://rationalwiki.org/wiki/RationalWiki:Community_Standards#Mission
 * About