Source : Webster's Revised Unabridged Dictionary (1913)

Flue \Flue\, n. [Cf. OF. flue a flowing, fr. fluer to flow, fr.
   L. fluere (cf. {Fluent}); a perh. a corruption of E. flute.]
   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.

   {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.