composition vs piece of music

composition and piece of music both are nouns.

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

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

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

composition (noun)	piece of music (noun)
the act of creating written works	a musical work that has been created
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