The actual definition is First world countries are allied with NATO, Second world countries with the Russian federation (formally with the USSR), Third world countries are unaligned with either, and Fourth world are unrecognized nations like the sentinelese and tribal nations within the US.
A strict application of this definition makes Switzerland and Sweden third world countries throughout the Cold War, so it’s not really something you can use to describe modern usage of the term
There’s no actual definition of these terms, it’s all arbitrary.
The actual definition is First world countries are allied with NATO, Second world countries with the Russian federation (formally with the USSR), Third world countries are unaligned with either, and Fourth world are unrecognized nations like the sentinelese and tribal nations within the US.
https://www.nationsonline.org/oneworld/third_world_countries.htm
A strict application of this definition makes Switzerland and Sweden third world countries throughout the Cold War, so it’s not really something you can use to describe modern usage of the term