First World or Developed World is a term used in geopolitics and economics to refer to the richer and more developed nations. In these countries, the majority of the people are in the middle class and enjoy a good standard of living. This term is being used less, as terms like global North and developed world become more common.

First World: the United States and its allies.
Second World: the Soviet Union, China, and their allies.
Third World: neutral and non-aligned countries.