The Collaborative International Dictionary of English v.0.48:

Basso \Bas"so\, n. [It., fr. LL. bassus. See Base, a.] (Mus.)
      (a) The bass or lowest part; as, to sing basso.
      (b) One who sings the lowest part.
      (c) The double bass, or contrabasso.
          [1913 Webster]

   Basso continuo. [It., bass continued.] (Mus.) A bass part
      written out continuously, while the other parts of the
      harmony are indicated by figures attached to the bass;
      continued bass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

basso
    n 1: an adult male singer with the lowest voice [syn: bass,
         basso]
    2: the lowest adult male singing voice [syn: bass, bass
       voice, basso]

Moby Thesaurus II by Grady Ward, 1.0:

49 Moby Thesaurus words for "basso":
   Heldentenor, Meistersinger, alto, aria singer, baritenor, baritone,
   bass, basso buffo, basso cantante, basso profundo, blues singer,
   canary, cantatrice, cantor, caroler, chanter, chantress,
   coloratura soprano, comic bass, contralto, countertenor, crooner,
   deep bass, diva, dramatic soprano, heroic tenor, hymner,
   improvisator, lead singer, lieder singer, melodist, mezzo-soprano,
   opera singer, prima donna, psalm singer, rock-and-roll singer,
   singer, singstress, songbird, songster, songstress, soprano, tenor,
   torch singer, vocalist, vocalizer, voice, warbler, yodeler