This article relies largely or entirely on a single source. (February 2021)
Parliamentary opposition is a form of political opposition to a designated government, particularly in a Westminster-based parliamentary system. This article uses the term government as it is used in Parliamentary systems, i.e. meaning the administration or the cabinet rather than the state. The title of "Official Opposition" usually goes to the largest of the parties sitting in opposition with its leader being given the title "Leader of the Opposition".
In first-past-the-post assemblies, where the tendency to gravitate into two major parties or party groupings operates strongly, government and opposition roles can go to the two main groupings serially in alternation.
The more proportional a representative system, the greater the likelihood of multiple political parties appearing in the parliamentary debating chamber. Such systems can foster multiple "opposition" parties which may have little in common and minimal desire to form a united bloc opposed to the government of the day.
Some well-organised democracies, dominated long-term by a single faction, reduce their parliamentary opposition to tokenism. Singapore exemplifies a case of a numerically weak opposition; South Africa under the apartheid regime maintained a long-term imbalance in the parliament. In some cases tame "opposition" parties are created by the governing groups in order to create an impression of democratic debate.
Some legislatures offer opposition parties particular powers. In Canada, the United Kingdom, and New Zealand 20 days each year are set aside as "Opposition Days" or "Supply Days", during which the opposition gets to set the agenda. Canada also has a Question Period, during which the opposition (and the Parliament generally) can ask questions of government ministers.