headcanon

As a noun headcanon

is (fandom slang) elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon.

As an adjective undefined is

lacking a definition or value.

As nouns the difference between headcanon and concept

is that headcanon is (fandom slang) elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon while concept is an understanding retained in the mind, from experience, reasoning and/or imagination; a generalization (generic, basic form), or abstraction (mental impression), of a particular set of instances or occurrences (specific, though different, recorded manifestations of the concept).

As nouns the difference between headcanon and idea

is that headcanon is (fandom slang) elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon while idea is (philosophy) an abstract archetype of a given thing, compared to which real-life examples are seen as imperfect approximations; pure essence, as opposed to actual examples.

As nouns the difference between headcanon and canon

is that headcanon is elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon while canon is a generally accepted principle; a rule.

As a noun headcanon

is (fandom slang) elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon.

As a verb conjecture is

.

As nouns the difference between headcanon and theory

is that headcanon is elements and interpretations of a fictional universe accepted by an individual fan, but not found within or supported by the official canon while theory is mental conception; reflection, consideration.