Party discipline is the ability of a parliamentary group of a political party to get its members to support the policies of their party leadership. In liberal democracies, it usually refers to the control that party leaders have over its legislature. Party discipline is important for all systems of government that allow parties to hold political power because it determines the degree to which the governmental infrastructure can be affected by legitimate political processes.
The term has a somewhat different meaning in Marxist-Leninist political systems such as the People's Republic of China. In this case it refers to administrative sanctions such as fines or expulsion that the Communist Party can impose on its members for actions such as corruption or disagreeing with the party.
Party discipline tends to be extremely strong in Westminster systems such as the United Kingdom, Canada, Australia, New Zealand and India in which a vote by the legislature against the government is understood to cause the government to "collapse," according to the convention of confidence votes. In these situations, it is extremely rare for a member to vote against the wishes of their party. Party leaders in such governments often have the authority to expel from the party members who violate the party line. Other examples of strong party discipline include the French Section of the Workers' International (SFIO) and the Communist Party.
In countries such as New Zealand, which use an MMP (mixed-member proportionality) system of voting, party discipline tends to be high. This is especially true for list MP's who do not represent an electorate. If they do not vote the party line they risk being demoted on the list in the next election and not returning to parliament.
Weak party discipline is usually more frequent in parties of notables than in populist parties. The French Radical-Socialist Party had no party discipline, neither did any of the right-wing parties during the Third Republic (1871-1940).
A weak party discipline is also found Congressional systems such as the United States Congress. In these legislatures, it is routine for members to cross party lines on a given vote, typically following the interests of their region or following other members of a borderline group within their party (for instance, the conservative Blue Dog Democrats and centrist and socially liberal Main Street Republicans).