Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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}]
     2: largest and lowest member of the violin family [syn: {bass
        fiddle}, {bull fiddle}, {double bass}, {contrabass}, {string
        bass}]