The Collaborative International Dictionary of English v.0.48:

Billabong \Bil"la*bong`\, n. [Native name.]
   In Australia, a blind channel leading out from a river; --
   sometimes called an anabranch. This is the sense of the
   word as used in the Public Works Department; but the term has
   also been locally applied to mere back-waters forming
   stagnant pools and to certain water channels arising from a
   source.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

billabong
    n 1: a stagnant pool of water in the bed of a stream that flows
         intermittently
    2: a branch of a river made by water flowing from the main
       stream only when the water level is high