A monoid (or semigroup) is a principal left ideal monoid if all left ideals of are left principal ideals in . Similarily, is a principal right ideal monoid if all right ideals of are right principal ideals. Finally, is a principal ideal monoid if it is both a principal left ideal monoid and a principal right ideal monoid.
Examples
A group is a principal ideal monoid whose only ideal is isomorphic to itself.