From Greg Letts,
Your Guide to Table Tennis / Ping-Pong.
FREE Newsletter. Sign Up Now!

Definition: In table tennis, the ball is hit back and forth over the net onto the opponent's side of the table by the players.

In 2000, the diameter of ITTF approved table tennis balls was increased to 40mm from 38mm.

Balls are usually made from celluloid, although other similar plastics material are allowed (but not popular among players). The weight of the ball must be 2.7g, and the ball for ITTF competitions must be white or orange, and have matt, not shiny, finish.

Needless to say (but I'll say it anyway), the ball should be spherical!