Aspects or material that are not officially part of the source material but that fans promote or think could logically exist or occur in the source material. Although sometimes used interchangeably with "fan canon" (or "fanon"), the term can also refer to an individual fan's ideas, as opposed to those that have gained widespread currency. That pairing must be head canon because I definitely don't remember seeing those two kiss in any episode of the series. Devon swears that there was a fourth wizard, but that's just his head canon.