no, undefined behavior not just behavior that is not covered by the language definition. undefined behavior is term of art largely taken from C/C++, basically meaning that correct programs are assumed not to have these behaviors. for example, see https://en.cppreference.com/w/c/language/behavior. the definition of ub is not "just marketing". many major security vulnerabilities stem from having ub (out of bounds access, use after free). the point of rust is pretty much that you have to try hard to have ub, whereas in c/c++, it's basically impossible to not have ub.

To add onto this, Rust actually does have UB, it’s just impossible to reach without unsafe. One “sharper” edge has is that it’s UB is much easier to trigger in unsafe than one might expect, so writing unsafe Rust actually requires more skill than C++, which is why you should be very very careful when reaching for it.