The Collaborative International Dictionary of English v.0.48:

Baseball \Base"ball"\, n.
   1. A game of ball, so called from the bases or bounds (four
      in number) which designate the circuit which each player
      must endeavor to make after striking the ball.
      [1913 Webster]

   2. The ball used in this game.
      [1913 Webster]

WordNet (r) 3.0 (2006):

baseball
    n 1: a ball game played with a bat and ball between two teams of
         nine players; teams take turns at bat trying to score runs;
         "he played baseball in high school"; "there was a baseball
         game on every empty lot"; "there was a desire for National
         League ball in the area"; "play ball!" [syn: baseball,
         baseball game]
    2: a ball used in playing baseball