The Collaborative International Dictionary of English v.0.48:

Strike zone \Strike" zone\, n. (Baseball)
   the area above home plate where, if a pitched ball passes
   through and is not hit by the batter, the pitch counts as a
   strike. Specifically, it is directly above home plate and at
   a height from the knees to the shoulders of the batter.
   [PJC]

WordNet (r) 3.0 (2006):

strike zone
    n 1: (baseball) the area over home plate between a batter's
         knees and shoulders through which a pitch must pass in
         order to be called a strike