Flue

(flū), n. [Cf. O. Fr. flue, a flowing, from fluer, to flow, fluie, from Lat. fluvius, river.]

1. An air-passage; especially, one for conveying smoke and flame from a fire; a vertical compartment or division of a chimney.
2. (Steam-boilers.) A passage surrounded by water, for the gaseous products of combustion, in distinction from tubes, which hold water, and are surrounded by fire. Small flues are called flue-tubes. [See Illust. of Boiler.]