To chemistry, we call a combustion reaction to the type of reaction that causes a flame. Combustion is a chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, high-temperature exothermic (heat releasing) redox (oxygen adding) which creates oxidized, often gaseous products in a mixture called smoke.
Combustion is an exothermic reaction in the form of heat and light that releases energy. This releases the full amount of energy from the fuel being reacted when a gas undergoes complete combustion.
Methane or octane combustion is exothermic, releasing energy.
CH4 + 2O2 → CO2 + 2H2O + energy
Incomplete combustion happens when an oxygen-free combustion reaction occurs. Incomplete combustion is often avoided because it releases less fuel than complete combustion and creates a poisonous gas called carbon monoxide.