From SKYbrary Wiki
In the context of a combustible fuel mixture, the auto-ignition temperature is the lowest temperature at which the fuel will spontaneously ignite in a normal atmosphere without an external source of ignition such as a flame or spark.
This temperature is sometimes referred to as the kindling point of the fuel. Raising the temperature of a fuel to its auto-ignition point provides the energy required to initiate the chemical reaction needed for combustion. The auto-ignition temperature for a given fuel decreases as the pressure increases or as the oxygen concentration increases.