A coalition is formed when, in order to secure a parliamentary majority, political parties unite to form a government.
In Westminster, coalition governments are more likely to be formed during times of national emergency, as in wartime, when the needs and safety of the nation are considered to be of more importance than party political considerations.
War-time coalitions were formed in 1915 and in 1940.
Peacetime coalition is more unusual, but the post-war circumstances of 1918 and the financial crash of 1931 ushered in governments comprising more than one political party to tackle what were perceived as national crises.
The 1931 government was styled a "National Government".
In 1977 the Labour Government came to an agreement with the Liberal Party to sustain itself in government but this fell short of Liberal participation in the administration.
The electoral system in Britain is such that one party usually has a Parliamentary majority, even if it is sometimes slim, so true coalitions are relatively uncommon.
In local government and in the devolved assemblies, cross-party co-operation of some sort has become much more common in recent years.
For instance, in July 2007 Plaid Cymru agreed to share power with Labour in the Welsh Assembly Government.
As part of the deal, Plaid leader Ieuan Wyn Jones became deputy to the Labour First Minister Rhodri Morgan.