First past the post
| It doesn't stop|
at the water's edge
First past the post (FPTP) is a system used in elections, in which the candidate with the most votes wins the election, even if they fail to win an absolute majority. Nations with FPTP are usually either one-party systems, dominant-party systems, or de facto two-party systems. While it is easy to understand, the absurdity of this method can be highlighted in areas where parties can win an election, despite receiving fewer votes than their opponents. In the US, this is due to the electoral college system; in the UK it's from voters voting for local constituency MPs (Members of Parliament). In the US, the candidate with the most electoral college votes (that is, votes are measured in how many states won rather than how many voters voted for the president) becomes the president; in the UK, the party with the most MPs forms the government. In both cases, the 'winner' may have received fewer votes than the 'loser'. This has happened in, for example, the United States in 2000, and 2016 and in the UK in 1951 and 1974.
It promotes strong government (ha!), generally with 2 major parties, an observation called Duverger's law. This is because unlike mixed member proportional voting (MMPV) or instant runoff voting (IRV), all the losing votes are discarded. Third parties tend to do very badly in first past the post. A common argument in favor of this method is that it's very difficult for a political extremist to win a national election or gain power in general since they require most of the vote, thus promoting more centrist politics, though this argument is questionable given some far from centrist parties in countries with this voting system.
Third parties that do emerge in FPTP tend to have a rather strong regional base, as evidenced by the Bloc Quebecois or the Scottish National Party, neither of whom run candidates outside of their regional base. Another issue is the disparity of voter support to political power. For example, in the 2010 UK election, the Liberal Democrats secured 23% of the vote, but didn't even get 9% of the House of Commons. In fact, even though they went from 22% in 2005, they actually LOST five seats from 2005.