Bolshevik

A member of the radical group within the Russian Social Democratic Labor Party, which, under Vladimir I. Lenin’s leadership, staged the Bolshevik Revolution. In March 1918, the Bolsheviks formed the Russian Communist Party (Bolshevik) and began calling themselves Communists (q.v.). That party was the precursor of the Communist Party of the Soviet Union (CPSU-- q.v.).

Source: Library of Congress