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Papua New Guinea, Thailand and South Korea were first world in 1975? Aside from S. Korea, which is now first world (it was third world or maybe second world in 1975), Thailand is probably second world now, while Papua is very much still third world.
Anyways, to OP's question, a third world country to me is a country with little infrastructure, very low HDI, great poverty, high crime, low education, etc.
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