Many of the standard headers define NULL, and it is described, for example,
in section 7.17 Common definitions <stddef.h> (pdf page 266):

  NULL

  which expands to an implementation-defined null pointer constant

and "null pointer constant" is described in section 6.3.2.3 Pointers
(pdf page 59):

  3  An integer constant expression with the value 0, or such an expression
     cast to type void *, is called a null pointer constant (55).
     If a null pointer constant is converted to a pointer type, the resulting
     pointer, called a null pointer, is guaranteed to compare unequal to a
     pointer to any object or function.
  ---
  55) The macro NULL is defined in <stddef.h> (and other headers)
      as a null pointer constant; see 7.17.


On Linux systems, the standard C headers include other non-standard
Unix-specific headers, like /usr/include/linux/stddef.h which defines NULL:

  #ifndef _LINUX_STDDEF_H

  #define _LINUX_STDDEF_H
  
  #undef NULL
  
  #if defined(__cplusplus)
  #define NULL 0
  #else
  #define NULL ((void *)0)
  #endif
  
  #endif