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