heater vs fastball

heater and fastball both are nouns.

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

As nouns, heater and fastball are synonyms defined as:

Other synonyms of heater include bullet, hummer, smoke.

heater (noun)	fastball (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