Glossary term
What is not defined (ReferenceError: x is not defined)?
not defined stands for ReferenceError: x is not defined.
The name was never given any memory at all. Different from undefined, which means the slot exists but has no value yet.
Learn it properly · JavaScript CoreLesson 6: Undefined vs Not DefinedThese sound like the same complaint — "there's nothing there" — but JavaScript treats them as two completely different situations, one safe and expected, the other a genuine error. Mixing them up is one of the most common small misunderstandings in the language.