[AlotOfReading]

Every language has defined behavior. It's what you expect to happen through a program's execution. Sometimes there will be multiple possibilities, but you can still define them regardless. Laying this out explicitly is the purpose of a standard.

Undefined behavior is everything else. C and C++ are relatively unique in that their standards explicitly say "combining these constructs in this way is undefined", and we call those cases explicit UB. There's also a larger universe of implicit UB that standards omit. Most (all?) languages have implicit UB, even if they lack the explicit stuff. What happens when you get ENOMEM is a common one.

Rust does something similar to C/C++ and lists a bunch of UB that's only possible with incorrect code in unsafe blocks. Correct code placed in an unsafe block remains defined, as does code without unsafe (up to compiler/language bugs).

[flomo]

Yeah, if I understand correctly, the Rust project has no intention to formally 'define' what unsafe actually does, so its very implicit. Could be anything... so it's the Does It Work? standard.