alto vs countertenor

alto and countertenor both are nouns.

Word	Noun	Adjective	Verb	Adverb
alto	Yes	Yes	No	No
countertenor	Yes	Yes	No	No

As adjectives, alto and countertenor are synonyms defined as:

alto and countertenor: of or being the highest male voice; having a range above that of tenor

As nouns, alto and countertenor are synonyms defined as:

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

alto (noun)	countertenor (noun)
the pitch range of the lowest female voice	the highest adult male singing voice
(of a musical instrument) the second highest instrument in a family of musical instruments	a male singer with a voice above that of a tenor
the lowest female singing voice
the highest adult male singing voice
a singer whose voice lies in the alto clef

alto (adjective)	countertenor (adjective)
(of a musical instrument) second highest member of a group	of or being the highest male voice; having a range above that of tenor
of or being the highest male voice; having a range above that of tenor
of or being the lowest female voice