The Collaborative International Dictionary of English v.0.48:

Phlogiston \Phlo*gis"ton\ (fl[-o]*j[i^]s"t[o^]n; 277), n. [NL.,
   fr. Gr. flogisto`s burnt, set on fire, fr. flogi`zein to set
   on fire, to burn, fr. flo`x, flogo`s, a flame, blaze. See
   Phlox.] (Old Chem.)
   The former hypothetical principle of fire, or inflammability,
   regarded by Stahl as a chemical element; it is now known to
   be nonexistent.
   [1913 Webster + PJC]

   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. This theory is now discredited and superseded by
         the theory of chemical reaction between oxidizable
         substances and oxidants as an explanation of combustion
         [1913 Webster + PJC]

WordNet (r) 3.0 (2006):

phlogiston
    n 1: a hypothetical substance once believed to be present in all
         combustible materials and to be released during burning