The term 'Copenhagen interpretation' suggests some definite set of rules for interpreting the mathematical formalism of quantum mechanics. However, no such text exists, apart from some informal popular lectures by Bohr and Heisenberg, which contradict each other on several important issues. It appears that the term "Copenhagen interpretation", with its more definite sense, was coined by Heisenberg in the 1950s, while criticizing "unorthodox" interpretations such as that of David Bohm. Before the book was released for sale, Heisenberg privately expressed regret for having used the term, due to its suggestion of the existence of other interpretations, that he considered to be "nonsense".