The Collaborative International Dictionary of English v.0.48:

Contralto \Con*tral"to\ (? or ?), n. [It., fr. contra + alto.
   See Alto.] (Mus.)
   (a) The part sung by the highest male or lowest female
       voices; the alto or counter tenor.
   (b) the voice or singer performing this part; as, her voice
       is a contralto; she is a contralto.
       [1913 Webster]

   Note: The usual range of the contralto voice is from G, below
         middle C, to the C above that; though exceptionally it
         embraces two octaves.
         [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Contralto \Con*tral"to\ (? or ?), a. (Mus.)
   Of or pertaining to a contralto, or to the part in music
   called contralto; as, a contralto voice.
   [1913 Webster]

WordNet (r) 3.0 (2006):

contralto
    adj 1: of or being the lowest female voice [syn: alto,
           contralto]
    n 1: a woman singer having a contralto voice
    2: the lowest female singing voice [syn: contralto, alto]