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]

The Collaborative International Dictionary of English v.0.48:

Alto \Al"to\, n.; pl. Altos. [It. alto high, fr. L. altus. Cf.
   Alt.]
   1. (Mus.) Formerly the part sung by the highest male, or
      counter-tenor, voices; now the part sung by the lowest
      female, or contralto, voices, between in tenor and
      soprano. In instrumental music it now signifies the tenor.
      [1913 Webster]

   2. An alto singer.
      [1913 Webster]

   Alto clef (Mus.) the counter-tenor clef, or the C clef,
      placed so that the two strokes include the middle line of
      the staff. --Moore.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

high-pitched \high-pitched\ adj.
   1. high in pitch or frequency; -- used of sounds and voices.
      Opposite of low. [Narrower terms: adenoidal, pinched,
      nasal; altissimo; alto; countertenor, alto;
      falsetto; peaky, spiky; piping; shrill, sharp;
      screaky, screechy, squeaking, squeaky, squealing;
      soprano, treble; sopranino; tenor]

   Syn: high.
        [WordNet 1.5]

   2. set at a sharp or high angle or slant; as, a high-pitched
      roof.

   Syn: steeply pitched, steep.
        [WordNet 1.5] high-power

WordNet (r) 3.0 (2006):

alto
    adj 1: of or being the lowest female voice [syn: alto,
           contralto]
    2: of or being the highest male voice; having a range above that
       of tenor [syn: countertenor, alto]
    3: (of a musical instrument) second highest member of a group;
       "alto clarinet or recorder"
    n 1: a singer whose voice lies in the alto clef
    2: the lowest female singing voice [syn: contralto, alto]
    3: the highest adult male singing voice [syn: countertenor,
       alto]
    4: (of a musical instrument) the second highest instrument in a
       family of musical instruments
    5: the pitch range of the lowest female voice