dancer vs professional dancer

dancer and professional dancer both are nouns.

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

As nouns, dancer and professional dancer are synonyms defined as:

Other synonyms of dancer include terpsichorean.

dancer (noun)	professional dancer (noun)
a performer who dances professionally	a performer who dances professionally
a person who participates in a social gathering arranged for dancing (as a ball)