Investigations describing incidents with electrochemical cells, in which the cell heats up and is either destroyed or severely damaged, causing damage to the immediate environment of the cell, are frequently referred to as “thermal runaway“. However, the term is used in most papers without a clear definition of what is meant. This article analyses in more detail what exactly could be meant by this term, and intends to offer a precision of the usage of the term “thermal runaway” which will help to find solutions for thermal runaways and their propagation and safe handling in an electrochemical system like a lithium-ion battery. In addition, the key role of a fail-safe battery management system in preventing a thermal runaway is discussed.