dieresis vs diacritic

dieresis and diacritic both are nouns.

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

As nouns, diacritic is a hypernym of dieresis; that is, diacritic is a word with a broader meaning than dieresis:

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

Other hypernyms of dieresis include diacritical mark.

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

dieresis (adjective)	diacritic (adjective)
	capable of distinguishing