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]