A developed country is a sovereign state with a mature economy and technologically advanced infrastructure compared to other nations. Several factors determine whether or not a country is developed, such as its political stability, gross domestic product (GDP), level of industrialization, social welfare programs, infrastructure, and the freedoms its citizens enjoy. Countries that are not quite yet developed are called developing countries.
The Human Development Index was developed by the United Nations to measure human development in a country. HDI is quantified by looking at a country's human development, such as education, health, and life expectancy. HDI is set on a scale from 0 to 1, and most developed countries have a score above .80. HDI can be used to determine the best countries to live in, as more developed countries typically offer their residents a higher quality of life.
Because there are so many factors to consider, defining what countries are developed can be a challenge. Moreover, it's possible for a country to be developed in the view of one institution, but not in the eyes of another.
For example, the United Nations classifies Turkey as a developed country thanks to its HDI of .807. The CIA World Factbook, G-20, and several other international organizations all agree. However, organizations including Dow Jones and the Financial Times Stock Exchange Group (FTSE) classify Turkey as still developing. This is likely due to factors such as Turkey's slightly elevated infant mortality rate (12 per 1,000) and its citizens' comparatively low life expectancy (75 years).
Released in December 2020, the United Nations Human Development Report 2020 ranks each country in the world based on its HDI ranking. For 2020, the top 10 most developed countries (and their HDI scores) were:
1 - Norway
2 - Ireland (tie)
2 - Switzerland (tie)
4 - Iceland (tie)
4 - Hong Kong, China (SAR)(tie)
6 - Germany
7 - Sweden
8 - Australia (tie)
8 - Netherlands (tie)
10 - Denmark
For the full HDI 2020 list, including the United States, Russia, China, and the United Kingdom, see below (any score .80 or higher is developed, and anything lower is developing).