diacritic vs breve

diacritic and breve both are nouns.

Word	Noun	Adjective	Verb	Adverb
diacritic	Yes	Yes	No	No
breve	Yes	No	No	No

As nouns, breve is a hyponym of diacritic; that is, breve is a word with a more specific, narrower meaning than diacritic:

diacritic: a mark added to a letter to indicate a special pronunciation
breve: a diacritical mark (U-shaped) placed over a vowel to indicate a short sound

Other hyponyms of diacritic include accent, accent mark, cedilla, circumflex, hacek, wedge, macron, tilde, umlaut, diaeresis, dieresis.

diacritic (noun)	breve (noun)
a mark added to a letter to indicate a special pronunciation	a diacritical mark (U-shaped) placed over a vowel to indicate a short sound

diacritic (adjective)	breve (adjective)
capable of distinguishing