Although the word canonical is sometimes used as a noun, it is most properly used as an adjective meaning "relating to or congruent with the principles of canon." Fanfiction that is considered canonical adheres to the writing style, characterizations, plot, themes, and minutiae of its canon so well that the work is a near-perfect fit in the universe that the original author created. Uncanonical or non-canonical fanfiction is far more common. Often "canonical/uncanonical" and "goodfic/badfic" are used interchangeably, but there is a difference. There can be uncanonical goodfic, for instance, if someone posts a well-written AU or parody.