Definedはマクロが定義されているかどうかをテストします

The special operator  defined  is used in `#if' and `#elif' expressions to test whether a certain name is defined as a macro. defined  name and  defined ( name )  are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise. Thus,  #if defined MACRO  is precisely equivalent to  #ifdef MACRO . defined  is useful when you wish to test more than one macro for existence at once. For example,

     #if defined (__vax__) || defined (__ns16000__)

would succeed if either of the names  __vax__  or  __ns16000__  is defined as a macro.
Conditionals written like this:

     #if defined BUFSIZE && BUFSIZE >= 1024

can generally be simplified to just  #if BUFSIZE >= 1024 , since if  BUFSIZE  is not defined, it will be interpreted as having the value zero.
If the  defined  operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine  defined  operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option -pedantic, since other compilers may handle it differently.