Source : Free On-Line Dictionary of Computing

functor
     
        In {category theory}, a functor F is an operator on types.  F
        is also considered to be a {polymorphic} operator on functions
        with the type
     
        	F : (a -> b) -> (F a -> F b).
     
        Functors are a generalisation of the function "{map}".  The
        type operator in this case takes a type T and returns type
        "list of T".  The map function takes a function and applies it
        to each element of a list.
     
        (1995-02-07)