Fire occurs during combustion (a chemical reaction between substances, usually including oxygen) and is typically observed as a hot bright shifting body of gas, or as the emission of light by a heated object or body.

A combustion reaction is rapid as energy is generated at a faster rate than the rate of energy loss to the surroundings; this causes the temperature of the reactants to increase and the reaction to accelerate.

Light emission from a flame is the result of the presence of excited particles and, usually, of charged atoms and molecules and of electrons. A flame is said to be self-propagating through space. The flame propagation may be explained by heat conduction and diffusion. In heat conduction, heat flows from the flame front (the area in a flame in where combustion occurs) to the inner cone, where the unburned mixture of fuel and air is located. When the unburned mixture is heated to its ignition temperature, it combusts in the flame front, and heat from that reaction again flows to the inner cone, creating a self-propagating cycle. In diffusion, a similar cycle begins when reactive molecules produced in the flame front diffuse into the inner cone and ignite the mixture. A mixture can support a flame within a certain range of fuel gas concentration. The range boundaries are called the lower and upper limits of flammability. Mixtures of natural gas and air, for example, will not propagate flame if the proportion of gas is less than about 4 percent or more than about 15 percent. At low concentration, though there maybe localized ignition there is insufficient energy provided to heat nearby gas layers to ignition temperature. As the partial pressure of the fuel gas increases the energy increases to a point that may ignite nearby fuel gas and propagate the flame. With increasing fuel gas partial pressure the heat capacity of the gas/air mixture increases, and hence the flame temperature decreases, as the fuel gas typically has a higher heat capacity than air. By increasing the fuel gas concentration beyond the upper limit of flammability the cooler flame cannot ignite nearby gas, and the flame ceases to be self-propagating.

There are two main classifications for flame. A premixed flame is a type where fuel and air mix prior to burning. A diffusion flame occurs where air diffuses into the fuel within the flame. A candle is a diffusion flame, whereas a Bunsen burner has a premixed flame.

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