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

V.E.R.A. -- Virtual Entity of Relevant Acronyms (June 2006):

ARB
       Architecture Review Board (OpenGL, manufacturer, org.)