Duct

n.
  1. Any tube or canal by which a fluid or other substance is conducted or conveyed.
  2. (Anat.) One of the vessels of an animal body by which the products of glandular secretion are conveyed to their destination.
  3. (Bot.) A large, elongated cell, either round or prismatic, usually found associated with woody fiber.
  4. Guidance; direction.

Ducts are classified, according to the character of the surface of their walls, or their structure, as annular, spiral, scalariform, etc.