Real Dictionary

Third World
The countries of Africa, Asia, and South America are sometimes referred to all together as the Third World, especially those parts that are poor, do not have much power, and are not considered to be highly developed.

Third World in American English
those countries and territories, chiefly of Africa and Asia, once thought of as being outside the worlds, or spheres of influence, of the Western capitalist nations and the Soviet bloc
noun: the less economically developed nations of the world, esp. those with widespread poverty

Third World in British English
noun: the less economically advanced countries of Africa, Asia, and Latin America collectively, esp when viewed as underdeveloped and as neutral in the East-West alignment

third-world in American English
of or like the Third World; specif., economically underdeveloped, politically unstable, etc.