what does "-nan" mean

[janderson]

Tower Ty wrote:
> It does not mean  "null" , it does not mean "0" , it does not mean "" so just what does it signify
> 
> Not a number is cryptic . If it is not a number then just what is it ?

It signifies that whatever was assigned to the value was "not a number". 
  That is the program had no idea how to compute or assign a value given 
the input.  Its really a sentinel to indicate error or un-assignment.

-Joel