USSR

n. official name of the former Soviet Union (union of 15 republics in eastern Europe and northern Asia which was formed in 1922 and dissolved in 1991)

The Union of Soviet Socialist Republics (abbreviated USSR, ; tr.: Soyuz Sovetskikh Sotsialisticheskikh Respublik, SSSR), also called the Soviet Union (Russian: Сове́тский Сою́з; tr.: Sovetskiy Soyuz), was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. It was often incorrectly, and sometimes intentionally incorrectly, referred to as Russia after its largest and dominant constituent state. From 1945 until its dissolution in 1991 — a period known as the Cold War—the Soviet Union and the United States of America were the two world superpowers that dominated the global agenda of economic policy, foreign affairs, military operations, cultural exchange, scientific advancements including the pioneering of space exploration, and sports (including the Olympic Games and various world championships).
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Noun
1. a former communist country in eastern Europe and northern Asia; established in 1922; included Russia and 14 other soviet socialist republics (Ukraine and Byelorussia an others); officially dissolved 31 December 1991
(synonym) Soviet Union, Russia, Union of Soviet Socialist Republics
(hypernym) country, state, land
(part-holonym) Eurasia
(part-meronym) Soviet Socialist Republic
(class) Russian Revolution, February Revolution

Union of Soviet Socialist Republics (now Commonwealth of Independent States)  For more possible definitions for USSR, click here
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SU (not used)