The history and use of the acronym ‘BCS’ (named after Bardeen, Cooper and Schrieffer) in the science of superconductivity is traced and analysed. It is shown that a number of different theories are labelled ‘BCS’. The confusion in the application of the term ‘BCS’ is shown to be common because the term ‘theory’ itself is not precisely defined in physics. Recommendations are given to physics readers and students on how to distinguish between various theories referred to as ‘BCS’. Contributions from (...) scientists in different laboratories to the creation and formation of the superconductivity theory are indicated. (shrink)