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 that does nothing with its input and produces no output)
The ASCII character code with code value 0 is called a null character, sometimes denoted as NUL. Also, Unicode character 0000 is Null, denoted, like ASCII convention, 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, null result
In radio electronics, a null is an area or vector at which the signal from two or more of the antenna elements in an antenna system cancels out almost entirely.
This can be an advantage, as nulls in the horizontal plane can be used to protect other transmitters from interference. If not carefully planned however, nulls can affect the receivability of a signal in any given place. Null fill in the vertical plane is used to prevent this.