I'm thinking of terms like Character Development, Character Progression, Character Growth, Character Regression, Narrative Arc, Main Character, Protagonist, Supporting Cast, Side Character, Deuteragonist, Antagonist
A lot of these terms have overlapping meanings, and sometimes spark debates online about what they mean. Depending on who is talking, character development is used in online discussions to refer to either:
a character possessing a clear or strong narrative arc
a round, or thoroughly fleshed out character
Another example where the terms seem to get muddy: Do you call Killua from the Japanese manga Hunter x Hunter a main character, or a side character? If he is a side character how is his status different from say Bisky from the same series (a character with less prominence than the "main circle" of characters). If he is a main character, how is he different from say Gon (the character who's perspective is the one we follow most of the time)
In the case of the former, I feel like there should be some authoritative source I can point to so as to clear up confusion when discussing writing with other people without it just being "because I said that's what it means". In the case of the latter, I am not sure what I should refer to these characters as myself.
I found the following answer for a question that is worded very similarly: Source for literary terms?
The problem with this answer is the glossary in question does not define the terms in question (the first thing I search for is character development)
Once upon a time every blog and dictionary you could find all agreed on the definition of character development, but over time more sources have been appearing for the looser (and perhaps lazy) definition. So I'm overall less trustful of writing blogs when they define "main character" and "side character", and they don't even seem to completely agree.
I also consulted "A DICTIONARY OF Literary Terms and Literary Theory: FIFTH EDITION J. A. CUDDON" and had similar results, no definition for character development