Thermal runaway
Thermal runaway is a self-feeding failure in which a battery cell heats up, and that heat triggers internal chemical reactions that release even more heat. In lithium cells it can lead to venting, fire or explosion, and it can spread from one cell to the next. Common triggers are overheating, overcharging, physical damage and internal short circuits.
Example: A phone battery that swells, smokes and bursts into flame after being left on a hot car dashboard has gone into thermal runaway.