introit vs piece of music

introit and piece of music both are nouns.

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

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

introit: a composition of vocal music that is appropriate for opening church services
piece of music: a musical work that has been created

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

introit (noun)	piece of music (noun)
a composition of vocal music that is appropriate for opening church services	a musical work that has been created