Answer:
In electoral politics, a third party is any party contending for votes that failed to outpoll either of its two strongest rivals (or, in the context of an impending election, is considered highly unlikely to do so). The distinction is particularly significant in two-party systems. In any case "third" is often used figuratively, as in "the third parties", where the intent, literally stated, is "the third and succeeding parties".
For instance, in the United Kingdom a third party is a national political party, other than the Conservatives and Labour, which has at least one member in the House of Commons. From 1922 to 2015, Liberal Democrats and its predecessor Liberals was the third party. Since 2015, it is used for the Scottish National Party (SNP). In Scotland, SNP has been the dominant parliamentary party beginning with the 2011 Scottish Parliament election, with the Conservative the next largest party and Labour becoming Scotland's third party ever since.
In the United States of America, there have been numerous "third parties". The largest since the mid-20th century are the Libertarian and Green Parties.
In Canada provinces such as Ontario, Prince Edward Island and Quebec, nearly 50+ active political parties exist throughout the nation with several of them both provincial and federal levels either elected, formed or joined during cross-flooring.
The term "third parties" is used mostly in countries with first-past-the-post voting systems, as those systems tend to create a two-party system, so that successful smaller parties are rare to stronger two-party systems like the United States.
Countries using proportional representation give little advantage to the largest two parties, so they tend to elect many parties. Therefore, in those countries, three, four, or more political parties are usually elected to legislatures. In such parliamentary systems, coalitions often include smaller parties; since they may participate in a coalition government, there is not a sharp distinction with a 'major' party. In two-party systems, on the other hand, only the major parties have a serious chance of forming a government. Similarly, in presidential systems, third-party candidates are rarely elected president.
In some categorizations, a party needs to have a certain level of success to be considered a third party. Smaller parties that win only a very small share of the vote and no seats in the legislature often are termed minor or fringe parties.
