The Collaborative International Dictionary of English v.0.48:

arb \arb\ n. [Shortened from arbitrageur.]
   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); shortened form of
   arbitrageur.

   Syn: arbitrageur
        [WordNet 1.5] Arbalest

WordNet (r) 3.0 (2006):

arb
    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]