Canon

In the context of fiction, the canon of a fictional universe comprises those novels, stories, films, etc. that are considered to be genuine (or "official"), and those events, characters, settings, etc. that are considered to have inarguable existence within the universe. Usually items that are considered canon come from the original source of the universe while non-canon material comes from adaptations or unofficial items. Generally, Expanded Universes are not considered canon, though there are exceptions which are considered near-canon, or in the case of Star Wars, the Expanded Universe is considered full canon. In layman's terms, one could basically say that something that is canon is something that "actually happened" in that universe.