The Collaborative International Dictionary of English v.0.48:

Bass viol \Bass" vi`ol\ (Mus.)
   A stringed instrument of the viol family, used for playing
   bass. See 3d Bass, n., and Violoncello.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bass viol
    n 1: viol that is the bass member of the viol family with
         approximately the range of the cello [syn: viola da
         gamba, gamba, bass viol]
    2: largest and lowest member of the violin family [syn: bass
       fiddle, bass viol, bull fiddle, double bass,
       contrabass, string bass]