from ISO/IEC 9899:2011 "Programming Languages -- C"
3.4.3
1 undefined behavior behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements
2 NOTE Possible 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), to terminating a translation or execution (with the issuance of a diagnostic message).
It doesn't look like a leap to go from this definition to that of ignoring the situation completely with unpredictable results.
