bullet vs fastball

bullet and fastball both are nouns.

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

As nouns, bullet and fastball are synonyms defined as:

Other synonyms of bullet include heater, hummer, smoke.

bullet (noun)	fastball (noun)
(baseball) a pitch thrown with maximum velocity	(baseball) a pitch thrown with maximum velocity
a projectile that is fired from a gun
a high-speed passenger train