A parliamentary system of government, in short Parliamentarism, is a multi-party form of government in which the executive branch (the Cabinet or the Privy Council) is formally dependent of the Parliament's acceptance. The Cabinet, or single members thereof, can be removed by the Parliament through a vote of non-confidence. In addition, the executive branch can dissolve the legislature and call extra-ordinary elections. There is no clear-cut separation between the Parliament (the legislature) and the other branches of government.
The leader of the executive Cabinet, the Prime Minister, is usually the head of government - at least in practice. In most parliamentary systems the Prime Minister and the members of Cabinet are also members of the legislature. The leader of the leading party in the Parliament is often appointed to Prime Minister.
Under the parliamentary system the roles of head of state and head of government are more or less separated. In most parliamentary systems, the head of state is generally a ceremonial position, often a monarch or president, however sometimes retaining duties without much political relevance, such as Civil Service appointments. In many (but not all) parliamentary systems, the head of state may have reserve powers which are usable in a crisis. In most cases however, such powers are either by convention or by constitutional rule only exercised upon the advice and approval of the head of government.
Parliamentary systems vary as to the degree to which they have a formal written constitution and the degree to which that constitution describes the day to day working of the government. They also vary as to the number of parties within the system and the dynamics between to the parties. Also, relations between the central government and local governments vary in parliamentary systems, they may be federal or unitary states. The term can also be used for governance in local governments. An example is the city of Oslo, which has an executive council as part of a parliamentary system.
The Westminster System is a particular type of parliamentary model that developed out of parliamentary democracy as practiced at Westminster in London and was promulgated as the dominant form of government throughout the British Empire which are now known mostly as Commonwealth countries. In this model the head of state has considerable reserve powers which, have been limited in practice by convention rather than explicit constitutional rule.