grandpa vs grandfather

grandpa and grandfather both are nouns.

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

As nouns, grandpa and grandfather are synonyms defined as:

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

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