grandparent vs granny

grandparent and granny both are nouns.

Word	Noun	Adjective	Verb	Adverb
grandparent	Yes	No	No	No
granny	Yes	No	No	No

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

Other hyponyms of grandparent include gramps, grandad, granddad, granddaddy, grandfather, grandpa, gran, grandma, grandmother, grannie, nan, nanna.

grandparent (noun)	granny (noun)
a parent of your father or mother	a reef knot crossed the wrong way and therefore insecure
	the mother of your father or mother
	an old woman