The Collaborative International Dictionary of English v.0.48:

Mezzo-soprano \Mez"zo-so*pra"no\, a. (Mus.)
   Having a medium compass between the soprano and contralto; --
   said of the voice of a female singer. -- n.
   (a) A mezzo-soprano voice.
   (b) A person having such a voice.
       [1913 Webster]

WordNet (r) 3.0 (2006):

mezzo-soprano
    n 1: a soprano with a voice between soprano and contralto [syn:
         mezzo-soprano, mezzo]
    2: the female singing voice between contralto and soprano [syn:
       mezzo-soprano, mezzo]