It isn't unique. That is the one and only definition of "undefined behavior" as a term of art. Many cases of UB in C are perfectly well defined at the implemen

It isn't unique. That is the one and only definition of "undefined behavior" as a term of art. Many cases of UB in C are perfectly well defined at the implementation level. Some are not, for reasons ranging from lack of a demand signal, to the need for an expensive runtime, to compiler writers wanting to squeeze out every last drop of performance.