Use of the term "Eastern Bloc" generally refers to the "communist states of eastern Europe." Sometimes, more generally, they are referred to as "the countries of Eastern Europe under communism". Many sources consider Yugoslavia to be a member of the Eastern Bloc. Others consider Yugoslavia not to be a member after it broke with Soviet policy in the 1948 Tito–Stalin split.
The term "Eastern Bloc" was sometimes used interchangeably with the term Second World, being considered one of the two main blocs of the Cold War, opposed by the Western Bloc. The Soviet-aligned members of the Eastern Bloc besides the Soviet Union are often referred to as "satellite states" of the Soviet Union.
Prior to this use of the term, in the 1920s, "Eastern bloc" was used to refer to a loose alliance of eastern and central European countries.
Other countries that were not Soviet Socialist Republics, not Soviet Satellite States or not in Europe were sometimes referred to as being in the Eastern Bloc, Soviet Bloc, or Communist Bloc, including:
In 1922, the RSFSR, the Ukrainian SSR, the Byelorussian SSR and the Transcaucasian SFSR, approved the Treaty of Creation of the USSR and the Declaration of the Creation of the USSR, forming the Soviet Union. Soviet leader Joseph Stalin, who viewed the Soviet Union as a "socialist island", stated that the Soviet Union must see that "the present capitalist encirclement is replaced by a socialist encirclement."