What is defined as spontaneous combustion?

Spontaneous combustion refers to the phenomenon where a substance ignites on its own without an external source of ignition. This can occur due to internal chemical reactions or the accumulation of heat within a material. For instance, organic materials like oily rags or compost can generate heat through chemical processes, leading to temperatures high enough to ignite the material without any obvious external spark or flame.

The other choices do not accurately capture the essence of spontaneous combustion. While a fire caused by friction or one that starts from an external source describes ignition that relies on an identifiable trigger, they do not manifest the self-igniting characteristic central to spontaneous combustion. The mention of a chemical reaction producing heat, while relevant to some contexts, does not specify the self-sustaining nature of the ignition process that distinguishes spontaneous combustion from other types of fires.