Source : Webster's Revised Unabridged Dictionary (1913)

Shotgun \Shot"gun`\, n.
   A light, smooth-bored gun, often double-barreled, especially
   designed for firing small shot at short range, and killing
   small game.

Source : WordNet®

shotgun
     n : firearm that is a double-barreled smoothbore shoulder weapon
         for firing shot at short ranges [syn: {scattergun}]
     [also: {shotgunning}, {shotgunned}]