Source : Webster's Revised Unabridged Dictionary (1913)

Phlogiston \Phlo*gis"ton\, n. [NL., fr. Gr. ? burnt, set on
   fire, fr. ? to set on fire, to burn, fr. ?, ?, a flame,
   blaze. See {Phlox}.] (Old Chem.)
   The hypothetical principle of fire, or inflammability,
   regarded by Stahl as a chemical element.

   Note: This was supposed to be united with combustible
         (phlogisticated) bodies and to be separated from
         incombustible (dephlogisticated) bodies, the phenomena
         of flame and burning being the escape of phlogiston.
         Soot and sulphur were regarded as nearly pure
         phlogiston. The essential principle of this theory was,
         that combustion was a decomposition rather than the
         union and combination which it has since been shown to
         be.

Source : WordNet®

phlogiston
     n : a hypothetical substance once believed to be present in all
         combustible materials and to be released during burning