Originally, as you noted in your quote from NPR, the third world meant those countries who were neither part of the western/NATO alliance or the communist bloc.
Because those countries tended to be poorer nations, the term came to mean countries which were developing economies, a term that can also cause confusion, but generally refers to those countries which are at lower economic and infrastructural development levels than the western countries. The line is, not too surprisingly, a blurry one. Are Russia and China developed or developing countries? The answer, to a large extent, depends on where you would look—there are parts of both countries which can match the West in their economic and infrastructural levels, but there are also parts which lack, e.g., paved roads, electricity, telecommunications access, indoor plumbing. Then again, you can point at parts of the U.S. (shrinking, but still present) which also meet this criteria.
Because the use of “third world” as a marker for poor country is an informal (and discouraged) use, it would be hard to identify an exact line to draw, but I’ll offer a few possibilities:
Infrastructure: It’s generally assumed that in a third world country you will lack many of the following:
- Indoor plumbing
- Clean drinking water
- Electricity
- Phone/internet¹ access
- Paved roads
- Trusted law enforcement
- Democratically elected government
Economics: Third world economies would be characterized by
- Subsistence-level work (usually agriculture, although it may also be low-skill manufacturing or even trash-picking) where the people are just able to make enough income to provide for the most basic level of material needs
- Low per capita incomes and expenses (making these countries attractive for low-skill manufacturing)
- High levels of wealth inequality: the elites of the nation will, despite the masses being in poverty, have sufficient wealth to enjoy Western levels of luxury
And perhaps most controversially, but I think behind the deprecation of the term Third World, is the idea that to most Westerners, there is an inextricable connection between a country being non-White and Third World which reveals a subtle racism at play in both the application of the term (you’ll notice, for example, that most people would resist applying the term Third World to, say, Portugal, Spain or Greece, or even Russia, but not hesitate to apply it to Uruguay, India or China) and to the perpetuation of the situation (it’s a lot easier to declare that poverty is “just the way it is” if it affects people who are safely other).
- This marks a case of the goalposts moving—a decade or so ago, it might have been merely phone access, then merely internet access, and now it’s likely high-speed internet access.