The Collaborative International Dictionary of English v.0.48:

Abeam \A*beam"\, adv. [Pref. a- + beam.] (Naut.)
   On the beam, that is, on a line which forms a right angle
   with the ship's keel; opposite to the center of the ship's
   side.
   [1913 Webster]

WordNet (r) 3.0 (2006):

abeam
    adv 1: at right angles to the length of a ship or airplane