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Null usually refers to that which has no value (nothingness). It originates from the Latin word nullus, meaning not any.
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.
However, null is used in many other computing disciplines (often interchangeably with nul or
nil):
/dev/null is a virtual
device on UNIX systems that does nothing with its input and produces no output. The
NUL device has similar functionality on DOS and Windows.
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 programming 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.
In the context of theoretical physics, the adjective
null is a synonym of the word light-like.
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