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