A flame is a self-sustaining propagation of a localized combustion zone at subsonic velocities
•Flame must be localized: flame occupies only a small portion of combustible mixture at any one time (in contrast to a reaction which occurs uniformly throughout a vessel)
•A discrete combustion wave that travels subsonically is called a deflagration
•Combustion wave may be also travel at supersonic velocities, called detonation
•Fundamental propagation mechanism is different in deflagrations and detonations