prime minister, the head of government in a country with a parliamentary or semipresidential political system. In such systems, the prime minister—literally the “first,” or most important, minister—must be able to command a continuous majority in the legislature (usually the lower house in a bicameral system) to remain in office.

A prime minister 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.