A monoid (or semigroup) is a left GCD monoid if for all elements , there is a unique minimal left ideals in generated by and . Similarily, right GCD monoid if for all elements , there is a unique minimal right ideals in generated by and . Finally, is a two-sided GCD monoid if it is both a left GCD monoid and a right GCD monoid, and is a GCD monoid if is a two-sided GCD monoid and for all elements the minimal left and right ideals generated by and coincide.