The Collaborative International Dictionary of English v.0.48:

Heater \Heat"er\ (h[=e]t"[~e]r), n.
   1. One who, or that which, heats.
      [1913 Webster]

   2. Any contrivance or implement, as a furnace, stove, or
      other heated body or vessel, etc., used to impart heat to
      something, or to contain something to be heated.
      [1913 Webster]

   3. A pistol or other carryable firearm; as, gunmen with their
      heaters bulging in their pockets. [slang]
      [PJC]

   Feed heater. See under Feed.
      [1913 Webster]

WordNet (r) 3.0 (2006):

heater
    n 1: device that heats water or supplies warmth to a room [syn:
         heater, warmer]
    2: (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:

74 Moby Thesaurus words for "heater":
   Bunsen burner, Dutch oven, automatic, bedpan, bloom heater,
   blowgun, blowpipe, brazier, brick oven, burner, calefactory,
   caliduct, car heater, cooker, cookery, defroster,
   dielectric heater, dielectric preheater, electric blanket,
   electric heater, electronic heater, element, firearm,
   fireless heater, flamethrower, foot warmer, forge, furnace,
   gas jet, gas log, gat, geyser, gun, handgun, heat lamp,
   heating duct, high-frequency heater, hot-water bag,
   hot-water bottle, hot-water heater, induction heater,
   infrared heater, infrared lamp, ingot heater, iron heater, jet,
   kerosene heater, kiln, musket, orchard heater, oven, peashooter,
   piece, pilot light, pistol, preheater, radiator,
   radio-frequency heater, register, repeater, revolver, rifle, rod,
   sawed-off shotgun, shooting iron, shotgun, six-gun, six-shooter,
   steam pipe, stove, sun lamp, tewel, tuyere, warmer