The Collaborative International Dictionary of English v.0.48:

Canto \Can"to\, n.; pl. Cantos. [It. canto, fr. L. cantus
   singing, song. See Chant.]
   1. One of the chief divisions of a long poem; a book.
      [1913 Webster]

   2. (Mus.) The highest vocal part; the air or melody in choral
      music; anciently the tenor, now the soprano.
      [1913 Webster]

   Canto fermo[It.] (Mus.), the plain ecclesiastical chant in
      cathedral service; the plain song.
      [1913 Webster]

WordNet (r) 3.0 (2006):

canto
    n 1: the highest part (usually the melody) in a piece of choral
         music
    2: a major division of a long poem

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "canto":
   Spenserian stanza, accompaniment, air, alto, antistrophe, aria,
   baritone, bass, basso continuo, basso ostinato, bassus, book,
   burden, cantus, cantus figuratus, cantus planus, chorus, continuo,
   contralto, couplet, descant, distich, drone, envoi, epode,
   figured bass, ground bass, heptastich, hexastich, lay, line,
   measure, melodia, melodic line, melody, monostich, note, octastich,
   octave, octet, ottava rima, part, pentastich, plain chant,
   plain song, prick song, quatrain, refrain, rhyme royal, septet,
   sestet, sextet, solo, solo part, song, soprano, soprano part,
   stanza, stave, strain, strophe, syllable, tenor, tercet,
   terza rima, tetrastich, thorough bass, treble, triplet, tristich,
   tune, undersong, verse, voice, voice part