motet vs piece of music

motet and piece of music both are nouns.

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

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

motet: an unaccompanied choral composition with sacred lyrics; intended to be sung as part of a church service; originated in the 13th century
piece of music: a musical work that has been created

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

motet (noun)	piece of music (noun)
an unaccompanied choral composition with sacred lyrics; intended to be sung as part of a church service; originated in the 13th century	a musical work that has been created