The Collaborative International Dictionary of English v.0.48:

Mezzo \Mez"zo\, a. [It., from L. medius middle, half. See Mid,
   a.] (Mus.)
   Mean; not extreme.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mezzo
    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]

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "mezzo":
   amidships, average, central, core, equatorial, equidistant,
   halfway, interior, intermediary, intermediate, mean, medial,
   median, mediocre, mediterranean, medium, mesial, mid, middle,
   middlemost, middling, midland, midmost, midships, midway,
   nuclear