A two-party system is a system where two major political parties dominate voting in nearly all elections at every level of government and, as a result, all or nearly all elected offices are members of one of the two major parties. Under a two-party system, one of the two parties typically holds a majority in the legislature and is usually referred to as the majority party while the other is the minority party. While the term two-party system is somewhat imprecise and has been used in different countries to mean different things, there is considerable agreement that a system is considered to be of a two-party nature when election results show consistently that all or nearly all elected officials belong to only one of the two major parties, such as in the United States. In these cases, the chances for third party candidates winning election to any office are remote, although it's possible for groups within the larger parties, or in opposition to one or both of them, to exert influence on the two major parties.

A party system is a concept in comparative political science concerning the system of government by political parties in a democratic country. The idea is that political parties have basic similarities: they control the government, have a stable base of mass popular support, and create internal mechanisms for controlling funding, information and nominations.

The concept was originated by European scholars studying the United States, especially James Bryce and Moisey Ostrogorsky, and has been expanded to cover other democracies.Giovanni Sartori devised the most widely used classification method for party systems. He suggested that party systems should be classified by the number of relevant parties and the degree of fragmentation. Party systems can be distinguished by the effective number of parties.

Two structures of party system have been identified in the European Parliament since its first universal direct election in 1979, albeit the main EU party groups remained the same:

Italian party systems are usually considered only since the foundation of the Italian Republic (1946) as pre-fascist parties lacked a wide popular base.