What is a "second world" country? That depends upon when in history the label is being applied. The term second world was first used following World War II, when countries were categorized as First World, Second World, or Third World countries by French demographer Alfred Sauvy in 1952 (though there is debate as to whether he invented the terms or adopted them from another source). First World countries were the democratic, capitalist countries allied with the United States and NATO. Second World countries were the communist, socialist countries of the Eastern bloc, led primarily by the Soviet Union and China, many of which signed the Warsaw Pact of 1955. Third World countries were those neutral countries not aligned with either power. These definitions lasted throughout the Cold War (approx. 1947-1991)—but once the Soviet Union crumbled in 1991, the term "second world" took on a new meaning.
| Afghanistan | Cuba | Laos | Romania |
| Albania | Czechoslovakia | Mongolia | Somalia |
| Angola | East Germany | Mozambique | South Yemen |
| Benin | Ethiopia | Nicaragua | Soviet Union |
| Bulgaria | Grenada | North Korea | Vietnam |
| China | Hungary | Poland | Yugoslavia (1945-1948 only) |
| Congo (People's Rep.) | Kampuchea (Cambodia) |
After the disintegration of the Soviet Union in the early 1990s, the “three worlds” terminology took on new meanings. Today, “First World” countries are those considered to be developed countries. “Second World” countries are former communist countries that are neither prosperous nor poverty-stricken. “Third World” countries and even Fourth World countries are considered to be the developing and least developed countries of the world. Therefore, a term that was originally used to describe political ideologies is now used to define economic states.
The Human Development Index (HDI) is one of the world's most accurate and respected measures of economic development. The HDI tracks dozens of indicators such as life expectancy, education, and per capita income, then aggregates them all into a single value between 0 (lowest) and 1 (highest). Each country is then categorized into one of four levels of human development based upon their HDI score:
The Second World countries from the Cold War era currently range from medium human development to very high human development in terms of HDI. For example, Slovenia has a 2021/22 HDI of .918, categorizing it as very high human development. In historical terms, Slovenia is a Second World country; however, it is considered a “First World country” under the modern definition. Kyrgyzstan has a 2021/22 HDI of .692, putting the country in the medium human development tier. This makes Kyrgyzstan a “Second World country” by both historical definition and by modern definition.
When categorizing countries as First World, Second World, Third World, or Fourth World, it is important to distinguish between the historical and modern definitions of the term.
Modern Second World countries have medium to very high Human Development Index scores (0.55 to 0.80). By this definition, 91 countries qualify as Second World.