An exothermic reaction front or wave in a gaseous medium. Consider a uniform body of gas in which an exothermic chemical reaction (that liberates heat) is initiated by raising the temperature to a sufficiently high level; the reaction is started by a localized release of heat, as by a sufficiently energetic spark, and then spreads from the point of initiation. If the reaction is relatively slow, the whole gas will be involved before the initial region has finished reacting. If, however, the reaction is relatively fast, the reaction zone will develop as a thin front or wave propagating into the unreacted gas, leaving fully reacted gas behind. If the front, in addition, shows luminosity (emission of light), the flame may be considered a classical example. However, perceptible emission of visible radiation is not essential to the definition, and some flames are indeed vanishingly faint under ordinary viewing conditions.