castrato

from WordNet (r) 3.0 (2006)

castrato
    n 1: a male singer who was castrated before puberty and retains
         a soprano or alto voice

from The Collaborative International Dictionary of English v.0.48

Castrato \Cas*tra"to\, n. [L., properly p. p. of castrare. See
   {Castrate}.]
   A male person castrated for the purpose of improving his
   voice for singing; an artificial, or male, soprano. --Swift.
   [1913 Webster]