The process of causing a substance to combust. Initiation of combustion is evidenced by a glow, a flame, or explosion. A substance will burn when the rate of heat gain due to oxidation exceeds the rate of heat loss. The ignition delay is the time to ignition (p. 193) from the instant reactants are mixed or exposed to a heat source. The ignition energy is the quantity of heat or electrical energy that must be absorbed by a substance to ignite. The ignition temperature is the lowest temperature at which a substance can sustain combustion in air or oxygen at a specified pressure. See combustion.