In computing (particularly, in programming), undefined value is a condition where an expression does not have a value semantically correct in the object language, although it is syntactically correct. An undefined value must not be confused with empty string, Boolean "false" or other "empty" (but defined) values. Depending on circumstances, evaluation to an undefined value may lead to exception or undefined behaviour, but in some programming languages undefined values can occur during a normal, predictable course of program execution.

Dynamically typed languages usually treat undefined values explicitly when possible. For instance, Perl has undef operator which can "assign" such value to a variable. In other type systems an undefined value can mean an unknown, unpredictable value, or merely a program failure on attempt of its evaluation. Nullable types offer an intermediate approach; see below.

The value of a partial function is undefined when its argument is out of its domain of definition. This include numerous arithmetical cases such as division by zero, square root or logarithm of a negative number etc. Another common example is accessing an array with an index which is out of bounds, as is the value in an associative array for a key which it does not contain. There are various ways that these situations are handled in practice:

In applications where undefined values must be handled gracefully, it is common to reserve a special null value which is distinguishable from normal values. This resolves the difficulty by creating a defined value to represent the formerly undefined case. There are many examples of this:

While dynamically typed languages often ensure that uninitialized variables default to a null value, statically typed values often do not, and distinguish null values (which are well-defined) from uninitialized values (which are not).

Some programming languages have a concept of exception handling for dealing with failure to return a value. The function returns in a defined way, but it does not return a value, so there is no need to invent a special value to return.

A variation on this is signal handling, which is done at the operating system level and not integrated into a programming language. Signal handlers can attempt some forms of recovery, such as terminating part of a computation, but without as much flexibility as fully integrated exception handling.

A function which never returns has an undefined value because the value can never be observed. Such functions are formally assigned the bottom type, which has no values. Examples fall into two categories: