What does "flue" mean?

Flue: 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. Flue boiler. See under Boiler. -- Flue bridge, the separating low wall between the flues and the laboratory of a reverberatory furnace. -- Flue plate (Steam Boiler), a plate to which the ends of the flues are fastened; -- called also flue sheet, tube sheet, and tube plate. -- Flue surface (Steam Boiler), the aggregate surface of flues exposed to flame or the hot gases.