In parliamentary systems fashioned after the Westminster system, the prime minister is the presiding and actual head of government and head of the executive branch. In such systems, the head of state or the head of state's official representative (often the monarch, president, or governor-general) usually holds a largely ceremonial position, although often with reserve powers.
The prime minister is often, but not always, a member of the Legislature or the Lower House thereof 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) that are constitutionally vested in the crown and may be exercised without the approval of parliament.
As well as being head of government, a prime minister may have other roles or posts—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 Defence (although there was then no Ministry of Defence) and in the current cabinet of Israel, Benjamin Netanyahu also serves as Minister of Communications, Foreign Affairs, Regional Cooperation, Economy and Interior.
The term prime minister in its French form, premier ministre, is attested in 17th Century sources referring to Cardinal Richelieu after he was named to head the royal council in 1624. The title was however informal and used alongside the equally informal principal ministre d'État ("chief minister of the state") more as a job description. After 1661, Louis XIV and his descendants refused to allow one of their ministers to be more important than the others, so the term was not in use.
The term prime minister in the sense that we know it originated in the 18th century in the United Kingdom when members of parliament disparagingly used the title in reference to Sir Robert Walpole. During the whole of the 18th Century, Britain was involved in a prolonged conflict with France, periodically bursting into all-out war, and Britons took outspoken pride in their "Liberty" as contrasted to the "Tyranny" of French Absolute Monarchy; therefore, being implicitly compared with Richelieu was no compliment to Walpole. Over time, however, the title became honorific and remains so in the 21st century.
The monarchs of England and the United Kingdom had ministers in whom they placed special trust and who were regarded as the head of the government. Examples were Thomas Cromwell under Henry VIII; William Cecil, Lord Burghley under Elizabeth I; Clarendon under Charles II and Godolphin under Queen Anne. These ministers held a variety of formal posts, but were commonly known as "the minister", the "chief minister", the "first minister" and finally the "prime minister".
The power of these ministers depended entirely on the personal favour of the monarch. Although managing the parliament was among the necessary skills of holding high office, they did not depend on a parliamentary majority for their power. Although there was a cabinet, it was appointed entirely by the monarch, and the monarch usually presided over its meetings.