narthex vs portico

narthex and portico both are nouns.

Word	Noun	Adjective	Verb	Adverb
narthex	Yes	No	No	No
portico	Yes	No	No	No

As nouns, portico is a hypernym of narthex; that is, portico is a word with a broader meaning than narthex:

narthex: portico at the west end of an early Christian basilica or church
portico: a porch or entrance to a building consisting of a covered and often columned area

narthex (noun)	portico (noun)
a vestibule leading to the nave of a church	a porch or entrance to a building consisting of a covered and often columned area
portico at the west end of an early Christian basilica or church