The Collaborative International Dictionary of English v.0.48:

Canal \Ca*nal"\, n. [F. canal, from L. canalis canal, channel;
   prob. from a root signifying "to cut"; cf. D. kanaal, fr. the
   French. Cf. Channel, Kennel gutter.]
   [1913 Webster]
   1. An artificial channel filled with water and designed for
      navigation, or for irrigating land, etc.
      [1913 Webster]

   2. (Anat.) A tube or duct; as, the alimentary canal; the
      semicircular canals of the ear.
      [1913 Webster]

   3. A long and relatively narrow arm of the sea, approximately
      uniform in width; -- used chiefly in proper names; as,
      Portland Canal; Lynn Canal. [Alaska]
      [Webster 1913 Suppl.]

   Canal boat, a boat for use on a canal; esp. one of peculiar
      shape, carrying freight, and drawn by horses walking on
      the towpath beside the canal.

   Canal lock. See Lock.
      [1913 Webster]

WordNet (r) 3.0 (2006):

canal
    n 1: (astronomy) an indistinct surface feature of Mars once
         thought to be a system of channels; they are now believed
         to be an optical illusion
    2: a bodily passage or tube lined with epithelial cells and
       conveying a secretion or other substance; "the tear duct was
       obstructed"; "the alimentary canal"; "poison is released
       through a channel in the snake's fangs" [syn: duct,
       epithelial duct, canal, channel]
    3: long and narrow strip of water made for boats or for
       irrigation
    v 1: provide (a city) with a canal [syn: canal, canalize,
         canalise]