NAN
From cppreference.com
                    
                                        
                    
                    
                                                            
                    | Defined in header  <math.h> | ||
| #define NAN /*implementation defined*/ | (since C99) | |
The macro NAN expands to constant expression of type float which evaluates to a quiet not-a-number (QNaN) value. If the implementation does not support QNaNs, this macro constant is not defined. 
[edit] See also
| (C99)(C99)(C99) | returns a NaN (not-a-number) (function) | 
| (C99) | checks if the given number is NaN (function) | 
| C++ documentation for NAN | |


