The Collaborative International Dictionary of English v.0.48:

Viola \Vi"o*la\, n. [It. See Viol.] (Mus.)
   An instrument in form and use resembling the violin, but
   larger, and a fifth lower in compass.
   [1913 Webster]

   Viola da braccio [It., viol for the arm], the tenor viol,
      or viola, a fifth lower than the violin. Its part is
      written in the alto clef, hence it is sometimes called the
      alto.

   Viola da gamba [It., viol for the leg], an instrument
      resembling the viola, but larger, and held between the
      knees. It is now rarely used.

   Viola da spalla [It., viol for the shoulder], an instrument
      formerly used, resembling the viola, and intermediate in
      size between the viola and the viola da gamba.

   Viola di amore [It., viol of love: cf. F. viole d'amour], a
      viol, larger than the viola, having catgut strings upon,
      and brass or steel wires under, the keyboard. These,
      sounding sympathetically with the strings, yield a
      peculiarly soft and silvery sound. It is now seldom used.
      [1913 Webster]

WordNet (r) 3.0 (2006):

viola da gamba
    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]