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

Moby Thesaurus II by Grady Ward, 1.0:

113 Moby Thesaurus words for "backstop":
   advocate, aegis, arch dam, arm guard, back, bamboo curtain, bank,
   bar, barrage, barrier, bear-trap dam, beaver dam, boom, breakwater,
   breastwork, brick wall, buffer, bulkhead, bulwark, bumper,
   champion, cofferdam, contraceptive, copyright, crash helmet,
   cushion, dam, dashboard, defense, dike, ditch, dodger, earthwork,
   embankment, face mask, fence, fender, finger guard, foot guard,
   fuse, gate, goggles, governor, gravity dam, groin, guard,
   guardrail, hand guard, handrail, hard hat, helmet,
   hydraulic-fill dam, insulation, interlock, iron curtain, jam,
   jetty, knee guard, knuckle guard, laminated glass, leaping weir,
   levee, life preserver, lifeline, lightning conductor,
   lightning rod, logjam, mask, milldam, moat, mole, mound, mudguard,
   nose guard, pad, padding, palladium, parapet, patent, pilot,
   portcullis, preventive, prophylactic, protective clothing,
   protective umbrella, rampart, roadblock, rock-fill dam, safeguard,
   safety, safety glass, safety plug, safety rail, safety shoes,
   safety switch, safety valve, screen, seat belt, seawall, shield,
   shin guard, shutter dam, side with, stone wall, sun helmet,
   umbrella, uphold, wall, weir, wicket dam, windscreen, windshield,
   work