Code behavior that is not specified by the programming language standard, meaning the program could do anything unpredictable when executed.
The term emerged from formal language specifications in the 1970s-80s, particularly C standards. 'Undefined' indicates the language specification deliberately doesn't define what should happen, leaving it to compiler and hardware implementation.
The C standard committee once joked that undefined behavior could 'make demons fly out of your nose' - they weren't kidding about the unpredictability! Modern compilers can optimize based on the assumption that undefined behavior never happens, sometimes making bugs disappear in debug mode but crash in release mode.
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