The Collaborative International Dictionary of English v.0.48:

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.
      [Webster 1913 Suppl.]

   2. In rounders, the player who stands immediately behind the
      striking base.
      [Webster 1913 Suppl.]

   3. In cricket, the longstop; also, the wicket keeper.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

backstop
    n 1: (baseball) the person who plays the position of catcher
         [syn: catcher, backstop]
    2: (baseball) a fence or screen (as behind home plate) to
       prevent the ball from traveling out of the playing field
    3: a precaution in case of an emergency; "he acted as a backstop
       in case anything went wrong"
    v 1: act as a backstop