The Collaborative International Dictionary of English v.0.48:

Violone \Vi`o*lo"ne\, n. [It. violone, augment. of viola a viol.
   See Viol.] (Mus.)
   The largest instrument of the bass-viol kind, having strings
   tuned an octave below those of the violoncello; the
   contrabasso; -- called also double bass. [Written also
   violono.]
   [1913 Webster]