The Collaborative International Dictionary of English v.0.48:

banjo \ban"jo\ n. [Formerly also banjore and banjer; corrupted
   from bandore, through negro slave pronunciation.]
   A stringed musical instrument having a head and neck like the
   guitar, and a circular body like a tambourine. It has five
   strings, and is played with the fingers and hands.
   [1913 Webster]

WordNet (r) 3.0 (2006):

banjo
    n 1: a stringed instrument of the guitar family that has long
         neck and circular body