What phenomenon occurs when a combustible material ignites spontaneously without a flame or spark?

The phenomenon when a combustible material ignites spontaneously without the presence of a flame or spark is referred to as autoignition. This process occurs when a material's temperature rises to the point where it can ignite due to its own heat without needing an external ignition source. Autoignition is a critical concept in fire science and building construction as it can lead to unanticipated fire outbreaks in certain environments, particularly in situations where materials are subjected to heat due to degradation, compression, or other physical changes.

While "spontaneous ignition" may seem similar, it generally refers to the same concept but is less commonly used in technical contexts. Piloted ignition involves an outside source initiating combustion, such as a match or flame, while thermal ignition may imply ignition that occurs as a result of heat sources yet may not encapsulate the essence of ignition without an external source. Thus, autoignition is the most accurate term for combustion without an igniting spark or flame.