Canon
Canon is traditionally a biblical term for what is accepted within the Christian faith. In gaming terms it means that a piece of fact usually associated with lore is officially supported by the creators of the lore/game.
Canon in Diablo
For example, Diablo I had an expansion called Diablo I: Hellfire, which is an official expansion, but not made by Blizzard. The D3 Team has chosen to exclude this official expansion from the canon part of the Diablo lore, so anything you read or did in the expansion material never happened.
Information that IS canon can also be retconned, to become outdated, and no longer canon.
Non-Canon Information
DiabloWiki is archiving both canon and non-canon lore, but will clearly mark if a part of the material here is not canon.
Examples
Here are a few examples of content on DiabloWiki that is NOT canon.
