piece of music vs musical composition

piece of music and musical composition both are nouns.

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

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

piece of music and musical composition: a musical work that has been created

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

piece of music (noun)	musical composition (noun)
a musical work that has been created	a musical work that has been created