alto clef vs viola clef

alto clef and viola clef both are nouns.

Word	Noun	Adjective	Verb	Adverb
alto clef	Yes	No	No	No
viola clef	Yes	No	No	No

As nouns, alto clef and viola clef are synonyms defined as:

alto clef and viola clef: a clef that puts middle C on the third line of a staff

alto clef (noun)	viola clef (noun)
a clef that puts middle C on the third line of a staff	a clef that puts middle C on the third line of a staff