dieresis vs diacritical mark

dieresis and diacritical mark both are nouns.

Word	Noun	Adjective	Verb	Adverb
dieresis	Yes	No	No	No
diacritical mark	Yes	No	No	No

As nouns, diacritical mark is a hypernym of dieresis; that is, diacritical mark 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
diacritical mark: a mark added to a letter to indicate a special pronunciation

Other hypernyms of dieresis include diacritic.

dieresis (noun)	diacritical mark (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