A special value used to indicate that a variable intentionally contains either no value or an unknown value. It is used particularly in relational databases, where expressions that include a null value will generally return either ‘null’ or ‘unknown’. This is distinct from all numeric and logical values and requires special handling. Null is also used in programming to indicate that a pointer does not contain an address. Using a null pointer as if it did contain an address will generally cause a runtime error.