piece of music vs composition

piece of music and composition both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of music	Yes	No	No	No
composition	Yes	No	No	No

As nouns, piece of music and composition are synonyms defined as:

Other synonyms of piece of music include musical composition, opus, piece.

piece of music (noun)	composition (noun)
a musical work that has been created	the act of creating written works
	musical creation
	something that is created by arranging several things to form a unified whole
	the way in which someone or something is composed
	the spatial property resulting from the arrangement of parts in relation to each other and to the whole
	an essay (especially one written as an assignment)
	a musical work that has been created
	art and technique of printing with movable type
	a mixture of ingredients