The Collaborative International Dictionary of English v.0.48:

grounder \grounder\ (ground"[~e]r), n. (Baseball)
   A batted ball that hits the ground before it leaves the
   infield, or the act of hitting a baseball along the ground;
   also called ground ball; -- contrasted with fly or fly
   ball.

   Syn: .
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

grounder
    n 1: (baseball) a hit that travels along the ground [syn:
         grounder, ground ball, groundball, hopper]