Crossing the floor
The term originates from the British House of Commons, which is configured with the Government and Opposition facing each other on rows of benches. An MP who switched parties would need to cross the floor. A notable example of the latter is Winston Churchill, who crossed the floor from the Conservatives to the Liberals in 1904, before later crossing back in 1924.
Voting against party lines
The term has passed into general use in other Westminster parliamentary democracies (such as Canada, Australia, New Zealand and South Africa) even if many of these countries have semicircular or horseshoe-shaped debating chambers and mechanisms for voting without Members of Parliament leaving their seats. In most countries, it is most often used to describe members of the government party or parties who defect and vote with the opposition against some piece of government-sponsored legislation — although this usage is not widespread in Canada, where the term's usage is restricted to the second definition.
In Australia, one of the major parties (the Australian Labor Party) requires its members to pledge their support for the collective decisions of the Caucus, which theoretically prohibits them from crossing the floor; however in practice some Labor members disregard this pledge despite the disciplinary action which may result. Amongst other parties, crossing the floor is rare although Senator Barnaby Joyce of the National Party of Australia crossed the floor 19 times under the Howard coalition government. Nonetheless, the record for crossing the floor in the Australian Parliament goes to Tasmanian Senator Sir Reg Wright, who voted against his own party (the Liberal Party of Australia) on 150 occasions.
In the United Kingdom, Canada, and other countries, the term is also used to describe leaving one's party entirely and joining another party, such as leaving an opposition party to support the government (or vice versa), or even leaving one opposition party to join another. In both Canada and the United Kingdom, the term carries only this meaning, and is not used for a simple vote against the party line on a bill.
In April 2006, the premier of Manitoba, Canada, Gary Doer (NDP), proposed a ban on crossing the floor of the Manitoba legislature. According to Mr Doer, this move came in response to "the concern some voters have expressed over the high-profile defections of three federal MPs from their parties in just over two years." The resulting legislation, which amended the provincial Legislative Assembly Act, mandated that Members of the Legislature who quit their political party to serve out the remainder of their term as independents.
An extraordinary example occurred in Alberta, Canada in December 2014 when Danielle Smith, the leader of the official opposition Wildrose Party, and eight of her MLAs crossed the floor together to join the governing Progressive Conservative Association of Alberta. 
- Floor crossing (South Africa)
- List of British politicians who have crossed the floor
- List of Canadian politicians who have crossed the floor
- Transformism for a similar concept in Italy
- Party switching for a similar concept
- Aisle (political term)
- "Crossing the floor in the Federal Parliament 1950 – August 2004". Research Note no. 11 2005–06. Australian Parliament. October 10, 2005.
- Debelle, Penelope (May 31, 2008). "Independently inclined". The Age.
- Macafee, Michelle (April 11, 2006). "Proposed reforms would ban floor-crossing in Man.". Canadian Press. Archived from the original on March 23, 2007.
- "The Elections Reform Act - Schedule E". Statutes of Manitoba. Manitoba Laws. 2006.