The term "First World" was first introduced by French demographer Alfred Sauvy in 1952* and used frequently throughout the Cold War. The term was originally used to describe mostly democratic/capitalist countries that were politically aligned with NATO and the United States. By comparison, "Second World" countries referred to mostly communist nations aligned politically with the Soviet Union. Finally, "Third World" countries referred to neutral countries that did not fit into either previous category. However, since the Soviet Union's collapse signaled the end of the Cold War in the early 1990s, the meanings of these terms have changed significantly.
Modern First World Countries
Today, the term "First World countries" is essentially interchangeable with "developed countries", and typically describes countries that are considered to have reached the upper echelon of advancement in several categories. First World countries have a high-functioning democracy with little risk of instability or insurgency. Their economies are stable, innovative, and typically capitalist. First World countries enjoy the most advanced technologies, the highest standards of living, and the greatest degree of political and cultural influence across the globe.
First World is a subjective term (especially since its shift in meaning), so there is no definitive list of First World countries. However, a few common criteria are generally necessary in order for a country to earn First World distinction. Economic factors such as gross domestic product (GDP) and the gross national product (GNP) play a huge role, and many evaluators also consider additional factors such as life expectancy and literacy rate.
One of those evaluators is the United Nations, whose Human Development Index is one of the most highly respected analyses of per-nation quality of life in the world. Countries with an HDI score of 0.800 or higher are considered to have "very high human development", which would roughly equate to a First World country. The most recent HDI rankings will appear in the table lower down on this page, including the 66 countries that qualified for the distinction in the 2020 report. Additional lists of what would be considered First World countries include the International Monetary Fund's list of advanced economies, the CIA World Factbook's list of developed countries, and the World Bank's list of high-income economies.
First World Countries by 2021/22 HDI:
The Original First World Countries
Under the original, 1950s Cold War-era definition of the term, any list of First World countries would have included NATO members the United States, the United Kingdom, France, Australia, Belgium, Canada, Denmark, Greece, Iceland, Italy, Luxembourg, Netherlands, Norway, Portugal, Turkey, and West Germany. Many historians would also have added non-NATO allies, including some or all of Australia, Iran, Iraq, Israel, Japan (perhaps surprisingly, considering the country fought for the Axis during WWII), New Zealand, Pakistan, Philippines, South Korea, Spain, and Thailand, as well as the neutral, but Western-aligned Austria, Ireland, and Sweden. That said, it is important to remember that such lists would be historic, and would not be applicable today.
Modern Second World and Third World Countries
Once the Cold War ended, the term "First World countries" morphed, taking on its current definition. "Third World countries" has also adopted a new definition, and has come to indicate what the United Nations terms "developing countries" and "least developed countries." (lists at the links). Conversely, after the communist bloc it originally represented dissolved, the term "Second World countries" has failed to evolve a new meaning and has largely fallen out of use. For the full list of countries and their respective HDI scores (those scoring 0.800 or higher would be First World countries), see the table below.
*Certain sources dispute Sauvy's statement that he invented the term himself. Some dissenters claim Charles De Gaulle said it first, while others claim that the United Nations had already begun using the term in a strictly economic sense in 1945.