A prime minister, premier or chief of cabinet is the head of the cabinet and the leader of the ministers in the executive branch of government, often in a parliamentary or semi-presidential system. Under those systems, a prime minister is not the head of state, but rather the head of government, serving under either a monarch in a democratic constitutional monarchy or under a president in a republican form of government. Under some presidential systems, such as South Korea and Peru, the prime minister is the leader or most senior member of the cabinet, not the head of government.