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]

Moby Thesaurus II by Grady Ward, 1.0:

104 Moby Thesaurus words for "canal":
   Eustachian tube, Fallopian tube, aqueduct, arroyo, bed, bottleneck,
   canalization, canalize, carve, chamfer, channel, chisel, conduit,
   corrugate, course, crack, creek bed, crimp, culvert, cut, dado,
   defile, dike, ditch, donga, dry bed, duct, emunctory, engrave,
   entrenchment, fistula, flume, flute, fosse, furrow, gash, goffer,
   gouge, groove, gulch, gully, gullyhole, gutter, ha-ha, headrace,
   incise, intestines, irrigation ditch, isthmus, kennel, meatus,
   moat, narrow, narrows, neck, nullah, ostium, oviduct, pass, pleat,
   plow, pore, rabbet, race, rifle, river bed, riverway, runnel, rut,
   salpinx, score, scratch, slit, sluice, spillbox, spillway, strait,
   streak, stream bed, streamway, striate, sunk fence, swash,
   swash channel, tailrace, thoracic duct, throat, trench, trough,
   ureter, urethra, vagina, vas, vessel, wadi, water carrier,
   water channel, water furrow, water gap, water gate, watercourse,
   waterway, waterworks, wrinkle



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CANAL. A trench dug for leading water in a particular direction, and
confining it.
     2. Public canals are generally protected by the law which authorizes
their being made. Various points have arisen under numerous laws authorizing
the construction of canals, which have been decided in cases reported in 1
Yeates, 430; 1 Binn. 70; 1 Pennsyl. 462; 2 Pennsyl. 517; 7 Mass. 169; 1
Sumu. 46; 20 Johns. 103, 735; 2 Johns. 283; 7 John. Ch. 315; 1 Wend. 474; 5
Wend. 166; 8 Wend. 469; 4 Wend. 667; 6 Cowen, 698; 7 Cowen, 526 4 Hamm. 253;
5 Hamm. 141, 391; 6 Hamm. 126; 1 N. H. Rep. 339; See River.