dancer vs belly dancer

dancer and belly dancer both are nouns.

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

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

dancer (noun)	belly dancer (noun)
a performer who dances professionally	a woman who performs a solo belly dance
a person who participates in a social gathering arranged for dancing (as a ball)