The Collaborative International Dictionary of English v.0.48:

ping-pong \ping"-pong`\, n. [Imitative.]
   1. An indoor modification of lawn tennis played with small
      bats, or battledores, and a very light, hollow, celluloid
      ball, on a large table divided across the middle by a net.
      Also called table tennis. [[originally a trade name]
      [Webster 1913 Suppl. +PJC]

   2. A size of photograph a little larger than a postage stamp.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

table tennis
    n 1: a game (trademark Ping-Pong) resembling tennis but played
         on a table with paddles and a light hollow ball [syn:
         table tennis, Ping-Pong]