The Collaborative International Dictionary of English v.0.48:

Bullet \Bul"let\, n. [F. boulet, dim. of boule ball. See Bull
   an edict, and cf. Boulet.]
   1. A small ball.
      [1913 Webster]

   2. A missile, usually of lead, and round or elongated in
      form, to be discharged from a rifle, musket, pistol, or
      other small firearm.
      [1913 Webster]

   3. A cannon ball. [Obs.]
      [1913 Webster]

            A ship before Greenwich . . . shot off her ordnance,
            one piece being charged with a bullet of stone.
                                                  --Stow.
      [1913 Webster]

   4. The fetlock of a horse.

   Note: [See Illust. under Horse.]
         [1913 Webster]

   Bullet tree. See Bully tree.

   Bullet wood, the wood of the bullet tree.
      [1913 Webster]

WordNet (r) 3.0 (2006):

bullet
    n 1: a projectile that is fired from a gun [syn: bullet,
         slug]
    2: a high-speed passenger train [syn: bullet train, bullet]
    3: (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:

77 Moby Thesaurus words for "bullet":
   alpha decay, atom-chipping, atom-smashing, atomic disintegration,
   atomic reaction, atomization, ball, bar shot, barrel, beta decay,
   bird shot, bombardment, bowshot, breeding, bucket, buckshot,
   cannon, cannon shot, cannonball, case shot, chain reaction,
   cleavage, crossbar shot, detonation, discharge,
   disintegration series, dissociation, duck shot, dumdum bullet,
   ejection, exchange reaction, expanding bullet, fission,
   fission reaction, fleet, fly, fusillade, gamma decay, grape,
   grapeshot, gun, gunfire, gunshot, highball, hotfoot, ionization,
   langrel shot, manstopping bullet, neutron reaction,
   nonreversible reaction, nuclear fission, nucleization, pellet,
   photodisintegration, potshot, proton gun, proton reaction,
   reversible reaction, rifle ball, rocket, round shot, salvo, shell,
   shot, shrapnel, slug, split shot, splitting the atom, spray,
   stimulation, stoneshot, target, tattoo, thermonuclear reaction,
   volley, whiz, zip