Jacek Yerka, Eruption 

canon is what happens in the text. e.g. merlin tries to poison morgana

a headcanon is a fact that you have personally made up as an elaboration of canon. it is your invention. e.g. i think morgana and gwen had a teenage lesbian situationship before the events of canon. or, i think modern morgana would love evanesence.

an interpretation is how one reads the existing text. it is not an invented fact nor is it a headcanon, it is a reading. e.g. i think merlin tries to poison morgana out of misplaced self-hatred. or, i think that one line was delivered with suppressed malice and shows a nuanced relationship.

fanon is the general fandom’s accepted headcanons and interpretations that elaborate or recontextualize canon. e.g. merlin is a badass waif and morgana is a bitch.