A rapid but persistent chemical reaction accompanied by the emission of light and heat. The reaction is self-sustaining, unless extinguished, to the extent that it continues until the fuel concentration falls below a minimum value. Most commonly, it results from a rapid exothermic combination with oxygen by some combustible material. Flame and heat also may result from a reaction involving an agent other than oxygen. Thus, certain reactive metals such as zinc will burn in an atmosphere of chlorine.