When I was younger, there were "first world" (i.e. rich) and "third world" (i.e poor) countries. This was a relic of cold war theory, but I never really heard about "second world" as a normal term.
Nowadays, the terms "emerging economies" and "developing nations" seem to be the appellation of choice. Is this merely historical accident, political correctness, or was there some agenda at work that has found this new terminology to better describe the present situation in the "Global South?"
The terms are ambiguous, and their general definition shifted considerably after the cold war. Originally the three worlds referred to the cold war alliances, with the first world being the US and its allies, the second world being the USSR and its allies and the third world being the neutral countries. In the following map, third world countries by the cold war definition are shown in green:
Switzerland is a typical third world country, isn't it? ;)
The cold war definitions became irrelevant after the collapse of the Soviet Union, and even more so as traditionally communist countries shifted to a more Western model. After the cold war the definitions shifted to reflecting the "worlds" broad economic status, and by that definition many of the cold war adversaries, including both the US and Russia, would be placed in the first world.
The change in definition, from political ideology to economy, contributed to the gradual disuse of "second world" that by the later definition would mean countries that are somewhere in between poverty and prosperity. Without as clear a line between first and second world countries, the term became more confusing than accurate.
Developed and developing country and emerging economy are also vague and broad terms, but their definitions are constant and don't carry the no longer relevant cold war baggage.
It's both a historical factor AND political correctness, which is obvious when you look at etymology on Wiki:
French demographer, anthropologist and historian Alfred Sauvy, in an article published in the French magazine L'Observateur, August 14, 1952, coined the term Third World, referring to countries that were unaligned with either the Communist Soviet bloc or the Capitalist NATO bloc during the Cold War.
His usage was a reference to the Third Estate, the commoners of France who, before and during the French Revolution, opposed priests and nobles, who composed the First Estate and Second Estate, respectively. Sauvy wrote, "Like the third estate, the Third World is nothing, and wants to be something." He conveyed the concept of political non-alignment with either the capitalist or communist bloc.
Since Communist Soviet block, and the Cold War with it, is officially over, grouping the countries by geopolitical strategic alignment no longer works.
Moreover, given Sauvy's analogy with "worthless" peasant estate from the point of view of French nobles, one can understand that the label is somewhat insulting, at best. Given the egalitarian slant of modern discourse, it definitely isn't even remotely politically correct.
Of course, as is usual with political correctness, people are now upset with the new "developing vs. developed world" terms as well, e.g. http://tailsteak.com/archive.php?num=159 . (somewhat offtopic - this is typical, because labels frequently have root in reality, and as long as you want to label to real-different things, you will earlier or later run into real - instead of semantic - differences; and it's the former that the people who strongly care about political correctness don't like, whether they realize it or not).
