- a political party in Germany advocating a form of social organization based on the economic and political ideology of Karl Marx.
- any of several European political parties advocating a gradual transition to socialism or a modified form of socialism by and under democratic processes.
- a U.S. political party, organized about 1897, that joined former members of the Socialist Labor party to form the Socialist party.
- (in Britain 1981–90) a centre political party founded by ex-members of the Labour Party. It formed an alliance with the Liberal Party and continued in a reduced form after many members left to join the Social and Liberal Democratic Party in 1988
- one of the two major political parties in Germany (formerly in West Germany), favouring gradual reform
- any of the parties in many other countries similar to that of Germany
