  In this paper, we reformulate the definition of the iterated function systems
(denoted by general IFSs in this paper) and show the existence and uniqueness
(in some sense) of the limit sets generated by the general IFSs, to unify the
definitions of the limit sets introduced before. Note that the general IFSs are
defined on (possibly unbounded) complete metric spaces and we instead assume a
``natural" condition of general IFSs to show the main result. To obtain the
main result, we apply techniques in the Banach fixed point theorem to the
general IFSs with the ``natural" condition. Besides, we consider an example of
general IFSs.
