The term "functor" is badly overloaded in general -- almost every language that uses the term has its own definition. In C++, it's an object that can be called as a function (by virtue of defining 'operator()') [0]. In Prolog, it's the head and arity of a term [1]. In ML, it's a function from structures to structures [2].
[0] http://www.stanford.edu/class/cs106l/course-reader/Ch13_Func...
[1] http://www.swi-prolog.org/pldoc/man?predicate=functor/3
[2] http://en.wikipedia.org/wiki/Standard_ML#Module_system