A prime minister is the most senior minister of cabinet in the executive branch of government in a parliamentary system. The position is usually held by, but need not always be held by, a politician. In many systems the Prime Minister selects and can dismiss other members of the cabinet, and allocates posts to members within the Government. In most systems they are the presiding member and chairperson of the cabinet. In a minority of systems, notably in semi-presidential system of government, a prime minister is the official who is appointed to manage the civil service and execute the directives of the President.

In parliamentary systems like the United Kingdom's or Australia's Westminster system, the prime minister is the presiding and actual head of the government and head of the executive branch. In such systems, the head of state or the head of state's official representative (the King, Queen, President, or Governor-General), although officially the head of the executive branch, in fact holds a ceremonial position. The Prime Minister is often, but not always, a member of parliament and is expected with other ministers to ensure the passage of bills through the legislature. In some monarchies the monarch may also exercise executive powers (known as the Royal Prerogative) which are constitutionally vested in the Crown and can be exercised without the approval of parliament.

As well as being Head of Government, a prime minister may have other roles or titles — the Prime Minister of the United Kingdom, for example, is also First Lord of the Treasury and Minister for the Civil Service. Prime ministers may take other ministerial posts — for example during the Second World War Winston Churchill was also Minister of Defense (although there was then no Ministry of Defense). Former Australian prime minister Gough Whitlam was famous for forming his cabinet entirely of himself and his deputy as soon as the overall result was beyond doubt at the 1972 federal election (see First Whitlam Ministry).

The Prime Minister is a novel by Anthony Trollope, first published in 1876. It is the fifth of the "Palliser" series of novels.