The Collaborative International Dictionary of English v.0.48:

fastball \fast"ball`\ n. (Baseball)
   a baseball thrown with maximum velocity.

   Syn: heater, smoke, hummer, bullet.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

fastball
    n 1: (baseball) a pitch thrown with maximum velocity; "he swung
         late on the fastball"; "he showed batters nothing but
         smoke" [syn: fastball, heater, smoke, hummer,
         bullet]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "fastball":
   bowl, cast, change of pace, change-up, chuck, chunk, curve,
   downcurve, fling, flip, forward pass, heave, hurl, incurve,
   knuckleball, lateral, lateral pass, lob, outcurve, pass, peg,
   pitch, put, screwball, serve, service, shot-put, shy, sinker,
   slider, sling, spitball, spitter, throw, toss, upcurve