vocalist vs hummer

vocalist and hummer both are nouns.

Word	Noun	Adjective	Verb	Adverb
vocalist	Yes	No	No	No
hummer	Yes	No	No	No

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

vocalist: a person who sings
hummer: a singer who produces a tune without opening the lips or forming words

vocalist (noun)	hummer (noun)
a person who sings	(baseball) a pitch thrown with maximum velocity
	a singer who produces a tune without opening the lips or forming words