dancer vs tap dancer

dancer and tap dancer both are nouns.

Word	Noun	Adjective	Verb	Adverb
dancer	Yes	No	No	No
tap dancer	Yes	No	No	No

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

dancer: a performer who dances professionally
tap dancer: a dancer who sounds out rhythms by using metal taps on the toes and heels of the shoes

dancer (noun)	tap dancer (noun)
a performer who dances professionally	a dancer who sounds out rhythms by using metal taps on the toes and heels of the shoes
a person who participates in a social gathering arranged for dancing (as a ball)