Parliamentary opposition in the context of Westminster parliamentary system

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.

A war cabinet is a committee formed by a government in a time of war to efficiently and effectively conduct that war. It is usually a subset of the full executive cabinet of ministers, although it is quite common for a war cabinet to have senior military officers and opposition politicians as members.

The Government frontbench in the Parliament of the United Kingdom, also known as the Treasury Bench, consists of the Cabinet and all other ministers.

Parliamentary opposition to the Government frontbench is provided by the Official Opposition frontbench and the Liberal Democrat frontbench team.

In Westminster and other parliamentary systems, a backbencher is a member of parliament (MP) or a legislator who occupies no governmental office and is not a frontbench spokesperson in the Opposition, being instead simply a member of the "rank and file".

The term dates from 1855. The term derives from the fact that they sit physically behind the frontbench in the House of Commons. A backbencher may be a new parliamentary member yet to receive a high office, a senior figure dropped from government, someone who for whatever reason is not chosen to sit in the government or an opposition spokesperson (such as a shadow cabinet if one exists), or someone who prefers to be a background influence, not in the spotlight.

The Official Opposition (French: Opposition officielle) is the largest party of the parliamentary opposition, which is composed of members of Parliament (MPs) who are not in government. Typically, it is the second-largest party in the House of Commons.

The Official Opposition is viewed as the caucus tasked with keeping the government in check. It is also generally viewed as the alternative government or "government in waiting". The Official Opposition maintains a shadow cabinet, with the leader of the Official Opposition at its head, of members of Parliament (MPs) and senators who often have the same portfolio areas of interest as actual ministers. The spokesperson for each portfolio is known as an opposition critic. In the event the government loses the confidence of the House or the Official Opposition party wins a general election, the party is ready to become the government.

The leader of the Official Opposition (French: chef de l'Opposition officielle) is the leader of the largest political party in the Legislative Assembly of Ontario that is not in government and is typically the second-largest party. The position is formally titled the leader of His Majesty's Loyal Opposition (French: chef de la loyale opposition de Sa Majesté); under the Westminster system, while the parliamentary opposition opposes the incumbent government, it remains loyal to the Crown and thus to Canada.

Marit Stiles has served as the leader of the Official Opposition since February 4, 2023. She leads the New Democratic Party (NDP), which has held the second largest number of seats in the Legislative Assembly since the 2018 provincial election.

The leader of the opposition (Irish: Ceannaire an Fhreasúra) in Ireland is a de facto term sometimes used to describe the politician who leads the largest party in the parliamentary opposition in Dáil Éireann, the house of representatives of the Oireachtas (the Irish parliament). In the Dáil, the leader of the opposition sits on the right-hand side of the Ceann Comhairle and directly opposite the Taoiseach. The role is not an official one and is not recognised in the Irish constitution, nor in legislation.

The leader of the opposition is, by convention, the leader of the largest political party in the Dáil that is not in government. Opposition leaders leading a political party with five members or more have full speaking rights under Dáil standing orders; smaller parties and independent politicians are allowed to speak less often.