Complex_I
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header  
<complex.h>
  | 
||
|   #define Complex_I /* unspecified */ 
 | 
(since C99) | |
The Complex_I macro expands to a value of type const float _Complex with the value of the imaginary unit.
[edit] See also
|    (C99) 
 | 
   the imaginary unit constant i  (macro constant)  | 
|    (C99) 
 | 
   the complex or imaginary unit constant i  (macro constant)  |