An inclosed passage way for establishing and directing a current of air, gases, etc.; an air passage; esp.:
(a)
A compartment or division of a chimney for conveying flame and smoke to the outer air.
(b)
A passage way for conducting a current of fresh, foul, or heated air from one place to another.
(c)
(Steam Boiler) A pipe or passage for conveying flame and hot gases through surrounding water in a boiler; distinguished from a tube which holds water and is surrounded by fire. Small flues are called fire tubes or simply tubes.