"Null" has different meanings depending on context.
Null is another name for the empty set, denoted by the symbol ∅. For this reason, some programming languages use
nil for the empty list or empty tuple. Lisp uses
nil to mean the empty list and the boolean value false.
Pointers and References
Null (or sometimes
None), in programming languages that support it, is the value of an uninitialized variable, a pointer that doesn't point to a meaningful memory address, or an object that fails to respond to any message.
Nullable references were invented by C.A.R. Hoare in 1965 as part of the Algol W language. Hoare later described his invention as a "billion-dollar mistake".
For more information, see Null pointer.
Agreeing with Hoare's sentiment, many languages don't have a special
null value, choosing to use optional types instead.
NULL as a special marker in SQL or a relational database stands in place of a value that is missing, or in a join means "no corresponding row." The operators
IS NULL and
IS NOT NULL are required for comparisons to a literal
null: other comparisons between
NULL and any value evaluate to "unknown."
For more information, see Null (SQL).
NUL is the name given to the character with ASCII code zero (
0) - i.e. hex
00. In some languages, notably C,
NUL is used to mark the end of a character string.
For more information, see Null character.