Canon has two mutually exclusive meanings in role-playing games, both relating to the truth or validity of lore from a setting. In general, canon refers to lore that has been reliably established without being reliably contradicted.
Canon is the full collection of lore contained in official publications about a certain setting that has not been officially rendered non-canon. Future official publications (and more third party publications) for a setting are expected to be consistent with previously published canon material about that setting, or else to explain or justify why they are not.
Official canon is useful for keeping published setting consistent over time and consistent in interpretation between the campaigns of different groups, but in general individual groups are free to adhere to or diverge from official canon as much as they like. This may be intentional, or simply because they do not have access to every single publication in which canon material was established.
Some lore can become non-canon, pseudo-canon, or alternate canon, depending on material released later and the editorial direction taken by the series.
A setting that intentionally avoids establishing canon material is anti-canon.
Campaign canon is the lore that is considered true in the game world of a specific campaign run by a specific group. This is likely to be, initially, a subset of the official canon (if any) and some homebrew setting material. As time passes, the canon of a campaign may shift according to the events that have transpired within the campaign.
The term canon comes from religious language, referring to the books and writings that make up authentic Scripture.
- Canon (fiction) at Wikipedia, the free encyclopedia.