A combustible substance can sustain a sufficiently rapid, heat-producing chemical reaction with a suitable oxidizing agent. In ordinary air this usually means combination with molecular oxygen, producing enough heat to maintain the reaction after ignition. A visible flame is not essential: carbon can glow as it undergoes combustion.
Combustibility depends on the surroundings and experimental conditions. A cold block of wax does not ignite merely because it meets air: heating must first provide combustible vapour and overcome the kinetic barrier to reaction. Finely divided iron burns much more readily than a massive bar because more surface is exposed per unit mass. Nor is oxygen universally required: molecular hydrogen can undergo combustion in chlorine. The expression “supports combustion” identifies the surrounding reactant in a particular experiment; it does not establish an absolute division between two kinds of chemical substance.
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