fastball vs heater

fastball and heater both are nouns.

Word	Noun	Adjective	Verb	Adverb
fastball	Yes	No	No	No
heater	Yes	No	No	No

As nouns, fastball and heater are synonyms defined as:

Other synonyms of fastball include bullet, hummer, smoke.

fastball (noun)	heater (noun)
(baseball) a pitch thrown with maximum velocity	(baseball) a pitch thrown with maximum velocity
	device that heats water or supplies warmth to a room