Null

In computer programming, null is a special value for a pointer (or reference) that is used to signify that the pointer's target is not a valid data element. Often, 0 (zero) is used as the null value, as most operating systems consider it an error to try to access such a low memory address. Null is used in many other disciplines, and is not just used for pointers in programming. (Example: /dev/null, a virtual device[?] on UNIX systems)

The ASCII character code with code value 0 is called a null character, sometimes denoted as NUL.

In some languages, such as LISP, the designation is called nil.

In many disciplines, the concept of null allows a three-valued logic, with null indicating 'unknown'.

Note that this is not the same as the definition of nothing because it is more related to computer science.

See also: null set.