diacritic vs dieresis

diacritic and dieresis both are nouns.

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

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

diacritic: a mark added to a letter to indicate a special pronunciation
dieresis: a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel

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

diacritic (noun)	dieresis (noun)
a mark added to a letter to indicate a special pronunciation	a diacritical mark (two dots) placed over a vowel to indicate that it does not form a diphthong with an adjacent vowel

diacritic (adjective)	dieresis (adjective)
capable of distinguishing