What is the phenomenon called when a material ignites without the application of external heat?

The phenomenon being described is best classified as spontaneous ignition. This term refers to the process where a material ignites on its own without an external heat source being applied. Spontaneous ignition occurs when the temperatures within a material reach a point where it can combust merely due to internal reactions, such as oxidation or the buildup of heat from microbial activity.

While "spontaneous heating" may seem related, it generally refers to the gradual increase in temperature in a material due to various factors, which may not necessarily lead to ignition. On the other hand, "flash point" is the lowest temperature at which a liquid can form an ignitable mixture in air, but it does not apply in this context since we’re discussing ignition without the application of external heat. Lastly, combustion itself is the chemical reaction of burning, requiring an initial heat source. Thus, the most precise term for ignition occurring without external input is indeed spontaneous ignition.