For questions about the combustion of an organic molecule or inorganic substance. Do not use this tag if your question is about the [stoichiometry] of a combustion reaction, or for the calculations of [thermodynamics] functions (like enthalpy change).
It is an exothermic process by which a substance is oxidized (by an oxidizing agent such as oxygen) to release a large amount of energy in the form of heat and often light.
Combustion is a reaction with complicated radical chain mechanisms, depending on substrates - combustible substance and oxidiser.
Combustible materials include most of the organic compounds (for example petrol and ethanol) and many inorganic ones (like ammonia and boranes), as also many elementary substances, like carbon, sulfur or more reactive metals.
Oxygen is most popular oxidizer for combustion, but also chlorine, fluorine, and compounds of these elements can be used.