diaeresis vs diacritical mark

diaeresis and diacritical mark both are nouns.

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

As nouns, diacritical mark is a hypernym of diaeresis; that is, diacritical mark is a word with a broader meaning than diaeresis:

diaeresis: 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 diaeresis include diacritic.

diaeresis (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