fantasia vs piece of music

fantasia and piece of music both are nouns.

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

As nouns, piece of music is a hypernym of fantasia; that is, piece of music is a word with a broader meaning than fantasia:

fantasia: a musical composition of a free form usually incorporating several familiar themes
piece of music: a musical work that has been created

Other hypernyms of fantasia include composition, musical composition, opus, piece.

fantasia (noun)	piece of music (noun)
a musical composition of a free form usually incorporating several familiar themes	a musical work that has been created