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

Moby Thesaurus II by Grady Ward, 1.0:

33 Moby Thesaurus words for "banjo":
   Dobro guitar, F-hole guitar, Spanish guitar, archlute, balalaika,
   banjo-uke, banjo-ukulele, banjo-zither, banjorine, banjuke,
   bass guitar, centerhole guitar, concert guitar, electric guitar,
   guitar, lute, mando-bass, mando-cello, mandola, mandolin,
   mandolute, mandore, oud, pandora, pandura, samisen, sitar,
   steel guitar, tamboura, theorbo, troubadour fiddle, uke, ukulele