dancer vs ballet master

dancer and ballet master both are nouns.

Word	Noun	Adjective	Verb	Adverb
dancer	Yes	No	No	No
ballet master	Yes	No	No	No

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

dancer: a performer who dances professionally
ballet master: a man who directs and teaches and rehearses dancers for a ballet company

dancer (noun)	ballet master (noun)
a performer who dances professionally	a man who directs and teaches and rehearses dancers for a ballet company
a person who participates in a social gathering arranged for dancing (as a ball)