A parliamentary system of government is a democratic form of government where the party or coalition of parties with the greatest representation in the parliament forms the government, and its leader becomes the prime minister or chancellor. The executive functions are exercised by members of the parliament appointed by the prime minister to the cabinet. The legislative function is conducted through a unicameral or bicameral parliament composed of members accountable to the people they represent. The head of state is usually a person distinct from the head of government. The defining characteristic of the parliamentary system is the supremacy of the legislative branch within the three functions of government - executive, legislative, and judicial - and the blurring or merging of the executive and legislative functions.

In a parliamentary system, the government must have the support of the parliament for it to enter and remain in office. Members of parliament may hold their positions during an established period between regularly scheduled elections. However, they can be turned out of office at any point between the periodic parliamentary elections if the government formed by the majority party loses the support of the majority of the legislative body.

The parliamentary system originated in Britain and was adopted in several of its former colonies. It is the dominant form of government in Europe, with 32 of its 50 sovereign states being parliamentarian. It is also common in the Caribbean, being the form of government of 10 of its 13 island states, and in Oceania. Elsewhere in the world, parliamentary countries are less common, but they are distributed through all continents, most often in former colonies of the British Empire that subscribe to a particular brand of parliamentarianism known as the Westminster system.