The Second World was one of the "Three Worlds" during the Cold War, which described those countries aligned with the Soviet Union and allies in the Warsaw Pact and Eastern Bloc. This grouping was directly opposed to the First World, which grouped countries aligned with the United States and its allies in NATO and the Western Bloc.
The term is also sometimes used to describe either 20th century socialist states or middle-income countries broadly, rather than based on Soviet alignment.
During the Cold War, the Three Worlds Model was used to rank the development of countries and their economies. First World countries were capitalist and industrial; they shared similar political and economic institutions, and retained influence over parts of the former colonial world. Second World countries generally advocated state socialism and shared certain characteristics such as centrally planned economic systems, single-party states, and medium income levels. The First World and the Second World were competing for political and economic influence over the Third World, which was neither aligned with the United States nor the Soviet Union in the Cold War.
The scope of the "Second World" has varied substantially over time, and across interpretations.
Though most socialist states remained aligned with the Soviet Union until either dissolved, the TitoâÂÂStalin, Sino-Soviet, and AlbanianâÂÂSoviet splits in the mid-20th century led to a substantial decline in Soviet influence. Thus, the geographical scope of the Soviet-led Second World shrunk significantly. However, many continued to include communist states which were not aligned with the Soviet Union in the "Second World," such as Yugoslavia, China, Albania, and the Khmer Rouge.
By 1991, upon the dissolution of the Soviet Union and after the fall of many communist regimes, defining the Second World around Soviet influence or socialism in general became obsolete in a post-Cold War context.
Subsequently, the colloquial meaning of the first, second, and third worlds changed from being based on political alignment to the US or USSR, to an economic definition. In particular, the second world has been applied as a synonym for middle-income countries, also known as newly industrialized countries, as those countries historically aligned with the Soviet Union or other communist states tended to be poorer and less industrialized than the Western-aligned first world (largely developed countries), but wealthier and more industrialized than the non-aligned third world (largely least developed countries).
This redefinition would therefore include non-communist, Western-aligned, and non-aligned states, such as the Russian Federation, India, Iran, Ukraine, Mexico, Brazil, and South Africa.
The three-world theory has been criticized as crude and relatively outdated for its nominal ordering, and some sociologists have instead used the words "developed", "developing", and "underdeveloped" as replacement terms for global stratification (which have been criticized as displaying a colonialist mindset); nevertheless, the three-world theory is still popular in contemporary literature and media. This might also cause semantic variation of the term between describing a region's political entities and its people.