grandfather vs grandpa

grandfather and grandpa both are nouns.

Word	Noun	Adjective	Verb	Adverb
grandfather	Yes	No	No	No
grandpa	Yes	No	No	No

As nouns, grandfather and grandpa are synonyms defined as:

Other synonyms of grandfather include gramps, grandad, granddad, granddaddy.

grandfather (noun)	grandpa (noun)
the father of your father or mother	the father of your father or mother