From Envirowiki

The Third world refers to the poorest countries in the world: mostly those colonised, or previously colonised, by Europeans, but also some parts of asia that were never really colonised. It is part of the four worlds model of economics.

The term "third world" can be seen as derogatory (in the same way as the term second-class citizen). Alternatives (that are also sometimes derogatory, or non-inclusive generalisations) include:

• Majority world (as opposed to the rich minority world)
• Global south (as opposed to Europe, North America)
• The East (as opposed to the West)