NULL is a special array reference value that refers to no array. It's used to indicate that an array reference currently points to nothing. The way you refer to NULL varies from language to language, but the concept is the same. In C++ and FORTRAN 77, 0 (numeral zero) is the value of the NULL array. In C, the preprocessor symbol NULL is the value for the NULL array. In Python, it's the special constant None, and in Java it's null. In Fortran 90, there is a function set_null, to initialize a pointer to the NULL value, and there is a logical function is_null to test whether an array is equal to NULL.