“Canon” refers to a specific set of narrative or artistic works. Any narrative work that falls under whatever subset you define is “canon” for that subset. It was originally used to define “great works” in something called the “western canon” and that includes lots of very old books that people think are important, like Shakespeare, Dante’s Inferno, Plato’s Republic, and things like that.
The modern usage of the word often refers to grouping certain events and stories by either author or publisher, set in the same universe. So things written by J.K. Rowling in the world of witches and wizards is in the “Harry Potter Canon” or things published by Marvel Studios and take place in the same area as Iron Man are “MCU Canon”
“Head Canon” refers to the act of inserting events and ideas into those universes without the original author/publisher confirming it.
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