The Eastern Bloc, also known as the Communist Bloc (Combloc), the Socialist Bloc, and the Soviet Bloc, was the coalition of communist states of Central and Eastern Europe, Asia, Africa, and Latin America that were aligned with the Soviet Union and existed during the Cold War (1947–1991). These states followed the ideology of Marxism–Leninism, in opposition to the capitalist Western Bloc. The Eastern Bloc was often called the "Second World", whereas the term "First World" referred to the Western Bloc and "Third World" referred to the non-aligned countries that were mainly in Africa, Asia, and Latin America but notably also included former pre-1948 Soviet ally Yugoslavia, which was located in Europe.

A communist state, also known as a Marxist–Leninist state, is a one-party state that is administered and governed by a communist party guided by Marxism–Leninism. Marxism–Leninism was the state ideology of the Soviet Union, the Comintern after Bolshevisation and the communist states within the Comecon, the Eastern Bloc, and the Warsaw Pact. Marxism–Leninism currently still remains the ideology of a few parties around the world. After its peak when many communist states were established, the Revolutions of 1989 brought down most of the communist states, however, it is still the official ideology of the ruling parties of China, Cuba, Laos, Vietnam, and to a lesser extent, North Korea. During most of the 20th century, before the Revolutions of 1989, around one-third of the world's population lived under communist states.