1
general::
Underdeveloped or developing countries, as in The conditions in our poorest rural areas resemble those in the third world. This expression originated in the mid-1900s, at first denoting those countries in Asia and Africa that were not aligned with either the Communist bloc nations or the non-Communist Western nations. Because they were for the most part poor and underdeveloped, the term was transferred to all countries with those characteristics, and later still to poorer groups within a larger prevailing culture.
American Heritage Idioms