The Collaborative International Dictionary of English v.0.48:

Duct \Duct\ (d[u^]kt), n. [L. ductus a leading, conducting,
   conduit, fr. ducere, ductum, to lead. See Duke, and cf.
   Douche.]
   1. Any tube or canal by which a fluid or other substance is
      conducted or conveyed.
      [1913 Webster]

   2. (Anat.) One of the vessels of an animal body by which the
      products of glandular secretion are conveyed to their
      destination.
      [1913 Webster]

   3. (Bot.) A large, elongated cell, either round or prismatic,
      usually found associated with woody fiber.
      [1913 Webster]

   Note: Ducts are classified, according to the character of the
         surface of their walls, or their structure, as annular,
         spiral, scalariform, etc.
         [1913 Webster]

   4. Guidance; direction. [Obs.] --Hammond.
      [1913 Webster]

WordNet (r) 3.0 (2006):

duct
    n 1: 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]
    2: a continuous tube formed by a row of elongated cells lacking
       intervening end walls
    3: an enclosed conduit for a fluid

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "duct":
   Eustachian tube, Fallopian tube, adit, aqueduct, canal, channel,
   conduit, course, ditch, egress, emunctory, entrance, exit, fistula,
   ingress, intestines, meatus, ostium, oviduct, passage, passageway,
   pore, salpinx, thoracic duct, trench, trough, troughing, troughway,
   tunnel, ureter, urethra, vagina, vas, vessel, watercourse, way