The Collaborative International Dictionary of English v.0.48:

Flue \Flue\ (fl[=u]), n. [Cf. OF. flue a flowing, fr. fluer to
   flow, fr. L. fluere (cf. Fluent); a perh. a corruption of
   E. flute.]
   1. 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.
          [1913 Webster]

   2. In an organ flue pipe, the opening between the lower lip
      and the languet.
      [Webster 1913 Suppl.]

   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.
      [1913 Webster]