Source : Webster's Revised Unabridged Dictionary (1913)

Backstop \Back"stop`\, n.
   1. In baseball, a fence, prop. at least 90 feet behind the
      home base, to stop the balls that pass the catcher; also,
      the catcher himself.

   2. In rounders, the player who stands immediately behind the
      striking base.

   3. In cricket, the longstop; also, the wicket keeper.

Source : WordNet®

backstop
     n : (baseball) a fence or screen (as behind home plate)
     v : act as a backstop