The Collaborative International Dictionary of English v.0.48:

Home run \Home" run`\, n. (Baseball)
   a complete circuit of the bases made by the batter without
   being put out and without an error on the play; also, the hit
   on which the batter makes such a circuit; a four-base hit.

   Note: Most home runs are made when the batter hits a fair
         ball out of the playing area on a fly, when the ball
         cannot be played by the defending team; if a batter
         circuits the bases while the ball is still inside the
         playing field, it is an inside-the-park home run.
         [PJC]