The poorer countries of the world — most of Africa, parts of Asia, parts of Central and South America. It means countries that are not rich, that do not have many big factories, where many people do not have enough money, good hospitals, or clean water. You will see it as a noun, almost always with 'the': 'aid to the Third World'. You will also see it in front of another noun: 'a Third World country', 'Third World debt'. Now the part your dictionary may not tell you. The name did not start as an insult, and it did not mean 'number three, the worst'. In the 1950s people talked about three groups: the West and its friends (the 'First World'), the Soviet Union and its friends (the 'Second World'), and everyone else — the countries that stayed out of that fight. That third group was the 'Third World'. Then the Soviet side ended, so there is no 'Second World' any more, and the name stopped making sense. What people hear now is only 'third = last = poor', so many people feel it is rude. Careful writers, and news reporters, now say 'developing countries', 'low-income countries', or 'the Global South' instead. And notice something else: it puts very different places in one box. Taiwan, South Korea, Ghana and Bolivia are not one thing. That is another reason people avoid it. So: you should understand this word, because you will meet it in older books, in films, and in speech. Use it yourself only carefully, and know that some listeners will not like it. There is one more use, and it is the rudest: people say 'this is so Third World' about a bad road, a power cut, or a slow office in a rich country. That is a complaint, it is unkind, and I would not say it.