4 Answers

It is a chemical reaction (exothermic=gives out heat) in which you "burn" a fuel.

Reactants of combustion:
fuel + oxygen

Complete combustion gives CO2
In-complete combustion gives CO2+CO (soot)

Testing for CO2:
- turns clear limewater milky

Testing for water:
- turns white custard powder yellow.

Combustion is a process that takes place when fuel, most commonly a fossil fuel, reacts with the oxygen in air to produce heat. In other words, it is the act or process of burning. Therefore it is the process of converting fuel into heat and this process requires oxygen.

is the sequence of the exothermic chemical reaction between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species 

WOW! These are some outstanding answers on Combustion.  Thumbsup for everyone. (prior to my post)