Definitions for "Null pointer"
Keywords:  pointer, anywhere, nothing, void, valid
The value that is obtained by converting the number 0 into a pointer, for example, (void *) 0. The C and C++ languages guarantee that this value will not match that of any legitimate pointer, so it is used by many functions that return pointers to indicate an error. X/Open.
a language concept whose particular internal value does not matter
a perfectly valid value for a pointer to have