antiphon vs Improperium

antiphon and Improperium both are nouns.

Word	Noun	Adjective	Verb	Adverb
antiphon	Yes	No	No	No
Improperium	Yes	No	No	No

As nouns, Improperium is a hyponym of antiphon; that is, Improperium is a word with a more specific, narrower meaning than antiphon:

antiphon: a verse or song to be chanted or sung in response
Improperium: (usually plural) one of a series of antiphons and responses expressing the remonstrance of Jesus Christ with his people, usually sung on Good Friday.

Other hyponyms of antiphon include gradual.

antiphon (noun)	Improperium (noun)
a verse or song to be chanted or sung in response	(usually plural) one of a series of antiphons and responses expressing the remonstrance of Jesus Christ with his people, usually sung on Good Friday.