Social Democratic Party

British centrist political party 1981–90, formed by members of Parliament who resigned from the Labour Party. The 1983 and 1987 general elections were fought in alliance with the Liberal Party as the Liberal/SDP Alliance. A merger of the two parties was voted for by the SDP in 1987, and the new party became the Social and Liberal Democrats, which became the Liberal Democrats, leaving a rump SDP that folded in 1990.