3 Terms and definitions [intro.defs]

3.63[defns.undefined]undefined behavior

behavior for which this document imposes no requirements
[Note 1: 
Undefined behavior may be expected when this document omits any explicit definition of behavior or when a program uses an incorrect construct or invalid data.
Permissible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message ([defns.diagnostic])), to terminating a translation or execution (with the issuance of a diagnostic message).
Many incorrect program constructs do not engender undefined behavior; they are required to be diagnosed.
Evaluation of a constant expression ([expr.const]) never exhibits behavior explicitly specified as undefined in [intro] through [cpp].
— end note]