The Collaborative International Dictionary of English v.0.48:

arbitrageur \arbitrageur\ n.
   someone who engages in arbitrage; i. e. one who purchases
   securities in one market for immediate resale in another in
   the hope of profiting from the price differential.

   Syn: arb, arbitrager.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

arbitrageur
    n 1: someone who engages in arbitrage (who purchases securities
         in one market for immediate resale in another in the hope
         of profiting from the price differential) [syn:
         arbitrageur, arbitrager, arb]