Although functors have been identified with function objects in print (Coplien?), calling "operator()", or an object that defines it, a "functor" is not universally agreed-upon usage with C++. As far as I know, the term functor has been used in mathematics for probably a century or longer for a mapping that maps functions to other things, for example to numbers. In programming terms, a functor would be a function whose argument type is a function type. Example:
typedef void (function_t)(); // Type of a function taking no args and
                             // returning void

int functor(function_t * const arg); // direct declaration of a function
                                     // that is a functor
Sure, functor may sound cooler than function object or operator(), but using it with this meaning unnecessarily ambiguates it. (Does this make me a pedant? ;-)