Definition of flue

• n. - An inclosed passage way for establishing and directing a current of air, gases, etc.; an air passage
• n. - A compartment or division of a chimney for conveying flame and smoke to the outer air.
• n. - A passage way for conducting a current of fresh, foul, or heated air from one place to another.
• n. - 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.
• n. - Light down, such as rises from cotton, fur, etc.; very fine lint or hair.