pontiff vs antipope

pontiff and antipope both are nouns.

Word	Noun	Adjective	Verb	Adverb
pontiff	Yes	No	No	No
antipope	Yes	No	No	No

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

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

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