flue

1. A pipe or duct that carries gaseous combustion products away from the point of combustion (such as a furnace).
2. An enclosed passageway in which to direct air or other gaseous current along.
3. A woolly or downy substance; down, nap; a piece of this.
4. In an organ flue pipe, the opening between the lower lip and the languet.

adjective

Shallow; flat