The Westminster system, or Westminster model, is a type of parliamentary government modelled on that of the Parliament of the United Kingdom. Key aspects of the system include an executive branch made up of members of the legislature which is responsible to the legislature; the presence of parliamentary opposition parties; and a ceremonial head of state who is separate from the head of government. The term derives from the Palace of Westminster, the seat of the British parliament. The Westminster system can be contrasted with the presidential system, which originated in the United States, and with the semi-presidential system based on the government of France.

The Westminster system is used, or was once used, in the national and subnational legislatures of most former colonies of the British Empire upon gaining self-government, beginning with the Province of Canada in 1848. However, many former colonies have since adopted other forms of government.

The Westminster system of government may include some of the following features:

Most of the procedures of the Westminster system originated with the conventions, practices, and precedents of the Parliament of the United Kingdom, which form a part of what is known as the Constitution of the United Kingdom. Unlike the uncodified British constitution, most countries that use the Westminster system have codified the system, at least in part, in a written constitution.

However, uncodified conventions, practices, and precedents continue to play a significant role in most countries, as many constitutions do not specify important elements of procedure. For example, some older constitutions using the Westminster system do not mention the existence of the cabinet or the prime minister, because these offices were taken for granted by the authors of these constitutions. Sometimes these conventions, reserve powers, and other influences collide in times of crisis and in such times the weaknesses of the unwritten aspects of the Westminster system, as well as the strengths of the Westminster system's flexibility, are put to the test. As an illustrative example, in the Australian constitutional crisis of 1975, the Governor-General of Australia, Sir John Kerr, dismissed Prime Minister Gough Whitlam and replaced him with opposition leader Malcolm Fraser.

The pattern of executive functions within a Westminster system is quite complex. In essence, the head of state, usually a monarch or president, is a ceremonial figurehead who is the theoretical, nominal or de jure source of executive power within the system. In practice, such a figure does not actively exercise executive powers, even though executive authority is nominally exercised in their name.

The head of government, usually called the prime minister or premier, will ideally have the support of a majority in the responsible house, and must, in any case, be able to ensure the existence of no absolute majority against the government. If the parliament passes a motion of no confidence, or refuses to pass an important bill such as the budget, then the government must either resign so that a different government can be appointed or seek a parliamentary dissolution so that new general elections may be held in order to re-confirm or deny the government's mandate.

Executive authority within a Westminster system is de jure exercised by the cabinet as a whole, along with more junior ministers, however, in effect, the head of government dominates the executive as the head of government is ultimately the person from whom the head of state will take advice (by constitutional convention) on the exercise of executive power, including the appointment and dismissal of cabinet members. This results in the situation where individual cabinet members in effect serve at the pleasure of the prime minister. Thus the cabinet is strongly subordinate to the prime minister as they can be replaced at any time, or can be moved ("demoted") to a different portfolio in a cabinet reshuffle for "underperforming".