What is defined as the moment when a mixture of fuel and oxygen encounters an external heat source sufficient to start combustion?

The moment when a mixture of fuel and oxygen encounters an external heat source sufficient to start combustion is known as piloted ignition. This occurs when a flame or another ignition source is applied to the fuel-oxygen mixture, allowing for combustion to begin in a controlled manner. Piloted ignition is critical in various applications, such as engines and burners, where an external source is necessary to initiate the chemical reactions leading to combustion.

In contrast, other terms refer to different ignition and combustion concepts. The flash point is the lowest temperature at which a liquid can vaporize to form an ignitable mixture. Spontaneous ignition refers to a situation where a substance ignites without any external heat source due to an increase in temperature from chemical reactions or pressure. Combustion initiation encompasses various processes, but specifically identifying it as piloted ignition indicates the reliance on an external heat source, which is the key aspect of the scenario described.