UndefinedSemantics are semantics that are undefined.
Standardised languages often have ``grey'' areas which it makes little or no sense to standardise these are said to be areas of UndefinedSemantics. Each implementation may do whatever it wishes when a programmer used a feature or combination of features with UndefinedSemantics.
native method calls have undefined semantics in Java, #pragma's are have undefined semantics in the C/C++ family of languages, many features of HTML tables have undefined semantics and perl? is non-standardised (I think...) and so consists entirely of UndefinedSemantics.
One page links to UndefinedSemantics:
