roller vs grounder

roller and grounder both are nouns.

Word	Noun	Adjective	Verb	Adverb
roller	Yes	No	No	No
grounder	Yes	No	No	No

As nouns, grounder is a hypernym of roller; that is, grounder is a word with a broader meaning than roller:

Other hypernyms of roller include ground ball, groundball, hopper.

roller (noun)	grounder (noun)
a grounder that rolls along the infield	(baseball) a hit that travels along the ground
Old World bird that tumbles or rolls in flight; related to kingfishers
pigeon that executes backward somersaults in flight or on the ground
a cylinder that revolves
a mechanical device consisting of a cylindrical tube around which the hair is wound to curl it
a small wheel without spokes (as on a roller skate)
a long heavy sea wave as it advances towards the shore