The Collaborative International Dictionary of English v.0.48:

Tether-ball \Teth"er-ball`\, n.
   A game played with rackets and a ball suspended by a string
   from an upright pole, the object of each side being to wrap
   the string around the pole by striking the ball in a
   direction opposite to the other.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

tetherball
    n 1: a game with two players who use rackets to strike a ball
         that is tethered to the top of a pole; the object is to
         wrap the string around the pole