Source : Webster's Revised Unabridged Dictionary (1913)

Plunger \Plun"ger\, n.
   1. One who, or that which, plunges; a diver.

   2. A long solid cylinder, used, instead of a piston or
      bucket, as a forcer in pumps.

   3. One who bets heavily and recklessly on a race; a reckless
      speculator. [Cant]

   4. (Pottery) A boiler in which clay is beaten by a wheel to a
      creamy consistence. --Knight.

   5. (Gun.) The firing pin of a breechloader.

   {Plunger bucket}, a piston, without a valve, in a pump.

   {Plunger pole}, the pump rod of a pumping engine.

   {Plunger pump}, a pump, as for water, having a plunger,
      instead of a piston, to act upon the water. It may be
      single-acting or double-acting

Source : WordNet®

plunger
     n 1: someone who risks losses for the possibility of considerable
          gains [syn: {speculator}]
     2: someone who dives (into water) [syn: {diver}]
     3: hand tool consisting of a stick with a rubber suction cup at
        one end; used to clean clogged drains [syn: {plumber's
        helper}]
     4: mechanical device that has a plunging or thrusting motion
        [syn: {piston}]