An undefined variable in the source code of a computer program is a variable that is accessed in the code but has not been declared by that code.

In computer programming, undefined behavior is the result of executing a program whose behavior is prescribed to be unpredictable, in the language specification of the programming language in which the source code is written.

In computing (particularly, in programming), undefined value is a condition where an expression does not have a correct value, although it is syntactically correct.