For me the definition was always:

• 1^st World = "Us", aka. NATO and Industrialized Countries.

• 2^nd World = "Them", aka. Warsaw Pact and Soviet Satellites.

• 3^st World = Nations militarily/economically irrelevant and/or support neither side during the Cold War.

For that reason with the fall of Socialism in late 1980s and the desperate July Coup to Restore Communism that destroyed the idea of a "Soviet Federation" democratic country, the World could no longer be divided by the previous method, and so was created the idea of:

• Developed Countries = former 1^st World
• Underdeveloped Countries = nations militarily/economically irrelevant
• Emergent Countries = former underdeveloped countries in fast economic development