diacritic vs umlaut

diacritic and umlaut both are nouns.

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

As nouns, umlaut is a hyponym of diacritic; that is, umlaut is a word with a more specific, narrower meaning than diacritic:

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

Other hyponyms of diacritic include accent, accent mark, breve, cedilla, circumflex, hacek, wedge, macron, tilde, diaeresis, dieresis.

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

diacritic (adjective)	umlaut (adjective)
capable of distinguishing