Undefined

An undefined value represents the absence of a value.

Related concepts:  ValueNull

Closest Wikipedia entry:  Undefined value

Misconceptions about Undefined

3 documented Misconceptions

AccessingInexistentPropertyError — JavaScript

Accessing a non existent property on an object produces an error

NoReturnValue — JavaScript

Functions without return statements return no value at all

NullAndUndefinedAreTheSame — JavaScript

The values null and undefined are the same