A prime minister, also known as a chief of cabinet, chief minister, first minister, minister-president or premier, 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. A prime minister is not the head of state, but rather the head of government, serving as the chief of the executive under either a monarch or a president in a republican form of government.
In parliamentary systems of government (be they constitutional monarchies or parliamentary republics), the Prime Minister (or occasionally a similar post with a different title, such as the Chancellor of Germany) is the most powerful politician and the functional leader of the state, by virtue of commanding the confidence of the legislature. The head of state is typically a ceremonial officer, though they may exercise reserve powers to check the Prime Minister in unusual situations.