Source : Webster's Revised Unabridged Dictionary (1913)

Pom-pom \Pom"-pom`\, n. [Imitative.]
   A Vickers-Maxim one-pounder automatic machine cannon using
   metallic ammunition fed from a lopped belt attached to the
   gun; -- popularly so called from its peculiar drumming sound
   in action. Sometimes, any of other similar automatic cannons.

Source : WordNet®

pom-pom
     n 1: decoration consisting of a ball of tufted wool or silk;
          usually worn on a hat [syn: {pompon}]
     2: artillery designed to shoot upward at airplanes [syn: {antiaircraft},
         {antiaircraft gun}, {flak}, {flack}, {ack-ack}, {ack-ack
        gun}]