'Phlogiston' definitions:

Definition of 'phlogiston'

noun

A hypothetical substance once believed to be present in all combustible materials and to be released during burning

From: GCIDE

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 This theory is now discredited and superseded by the theory of chemical reaction between oxidizable substances and oxidants as an explanation of combustion [1913 Webster + PJC]