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