antipope vs Roman Catholic Pope

antipope and Roman Catholic Pope both are nouns.

Word	Noun	Adjective	Verb	Adverb
antipope	Yes	No	No	No
Roman Catholic Pope	Yes	No	No	No

As nouns, Roman Catholic Pope is a hypernym of antipope; that is, Roman Catholic Pope is a word with a broader meaning than antipope:

antipope: someone who is elected pope in opposition to another person who is held to be canonically elected
Roman Catholic Pope: the head of the Roman Catholic Church

Other hypernyms of antipope include Bishop of Rome, Catholic Pope, Holy Father, pontiff, pope, Vicar of Christ.

antipope (noun)	Roman Catholic Pope (noun)
someone who is elected pope in opposition to another person who is held to be canonically elected	the head of the Roman Catholic Church