umlaut vs diacritic

umlaut and diacritic both are nouns.

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

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

Other hypernyms of umlaut include diacritical mark.

umlaut (noun)	diacritic (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

umlaut (adjective)	diacritic (adjective)
	capable of distinguishing