The strike zone is an imaginary box above home plate, and the umpire judges whether or not a baseball has entered it. His judgement can cause controversy.
The dimensions of the strike zone vary according to the batter's height and stance. The 2004 Official Rules of Major League Baseball defines it as "that area over home plate the upper limit of which is a horizontal line at the midpoint between the top of the shoulders and the top of the uniform pants (of the batter), and the lower level is a line at the hollow beneath the knee cap." Needless to say, his judgment can arouse controversy.