Monarch
A monarch is a hereditary ruler, figurehead or head of state, except in certain states like the former Kingdom of Poland and the Kalmar Union which featured an elective monarchy, and current-day Malaysia (see Yang di-Pertuan Agong) which uses a system of rotational selection from regional hereditary rulers.
A nation or state that is ruled by a monarch is called a kingdom. A system of governance involving a monarch is known as a monarchy or a constitutional monarchy.