The Collaborative International Dictionary of English v.0.48:

Martingale \Mar"tin*gale\, Martingal \Mar"tin*gal\, n. [F.
   martingale; cf. It. martingala a sort of hose, martingale,
   Sp. martingala a greave, cuish, martingale, Sp. alm['a]rtaga
   a kind of bridle.]
   1. A strap fastened to a horse's girth, passing between his
      fore legs, and fastened to the bit, or now more commonly
      ending in two rings, through which the reins pass. It is
      intended to hold down the head of the horse, and prevent
      him from rearing.
      [1913 Webster]

   2. (Naut.) A lower stay of rope or chain for the jib boom or
      flying jib boom, fastened to, or reeved through, the
      dolphin striker. Also, the dolphin striker itself.
      [1913 Webster]

   3. (Gambling) The act of doubling, at each stake, that which
      has been lost on the preceding stake; also, the sum so
      risked; -- metaphorically derived from the bifurcation of
      the martingale of a harness. Called also Martingale
      strategy. Such a betting strategy does not change the
      overall likelihood of winning, but in a short run it
      increases the probability of winning a small sum,
      balancing it against an increased probability of losing a
      large sum. [Cant] --Thackeray.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

martingale
    n 1: a harness strap that connects the nose piece to the girth;
         prevents the horse from throwing back its head
    2: spar under the bowsprit of a sailboat [syn: dolphin
       striker, martingale]