Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

billabong
     n 1: a stagnant pool of water in the bed of a stream that flows
          intermittently
     2: a branch of a river running to a dead end