"Undefined behavior" is a term of art[0], with the specific meaning as mentioned in the article: The compiler is allowed to assume that UB never happens, and ch

"Undefined behavior" is a term of art[0], with the specific meaning as mentioned in the article: The compiler is allowed to assume that UB never happens, and change the compiled code based on that assumption. Not "no one has written down a definition" or anything else. As the sibling comment points out, a contrasting term is "implementation-defined". Confusing the two is a common misconception when learning C++: You might expect "overflow is undefined behavior" to mean that there is no prescribed result and every compiler might do it differently, but each will produce results in its own consistent way. But that would be implementation-defined behavior; instead by doing unchecked addition, you tell the compiler that you know the addition will never overflow, and don't care at all about the overflowing case. [0] aka. "improper noun", see https://news.ycombinator.com/item?id=32673100 https://news.ycombinator.com/item?id=32673100