Null is a term used, often in computer science, to signify nothingness.
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)
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.
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.