umlaut vs diacritical mark

umlaut and diacritical mark both are nouns.

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

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

umlaut: a diacritical mark (two dots) placed over a vowel to indicate a change in sound in some languages
diacritical mark: a mark added to a letter to indicate a special pronunciation

Other hypernyms of umlaut include diacritic.

umlaut (noun)	diacritical mark (noun)
a diacritical mark (two dots) placed over a vowel to indicate a change in sound in some languages	a mark added to a letter to indicate a special pronunciation