father vs pa

father and pa both are nouns.

Word	Noun	Adjective	Verb	Adverb
father	Yes	No	Yes	No
pa	Yes	No	No	No

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

Other hyponyms of father include dad, dada, daddy, papa, pappa, pop, father-in-law, old man, pater.

father (noun)	pa (noun)
a male parent (also used as a term of address to your father)	an informal term for a father; probably derived from baby talk
the founder of a family
a person who holds an important or distinguished position in some organization
a person who founds or establishes some institution
the head of an organized crime family

father (verb)	pa (verb)
make (offspring) by reproduction