Hi Ann - that's an excellent question! In fact, it's the English interpretation of 'third' that's "not correct" and the subject of some controversy. 

A French demographer, Alfred Sauvy, coined the term "tiers monde" in an article published in the French magazine L'Observateur in 1952. The idea was that the world was divided into three parts, so "third" was indeed used in the sense of a fraction. The English interpretation however, has shifted over time, and become associated more with the ordinal sense of "third" as a position in some socioeconomic hierarchy, rather than the cold war political alliance groupings that gave the term its origin.

Hope that's helpful!

Gruff

Note: The first third referred to NATO countries and their allies, the second third referred to Russia and communist bloc and allies, the final third was all other countries.

Hi Alan, thanks for pointing that out. You're correct about the relation to "third estate" and that actually made me think quite hard about the nuances here in both languages. I agree that "fraction" isn't really the interpretation but then the ordinal third isn't quite right either. The point about the controversy is that, in English at least, "third" came to be heard as a ranking of importance, rather than the third of three distinct categories (unranked, but rather enumerated).

Although we only have one word in English, I think we still have the same nuance of a rank/order versus an enumeration.

It's the same enumeration sense that we use in "third party" (as opposed to "third place", which is a rank). 

Does that make sense?

Hi , Just to add to the debate ...

This is a expression dating from a post-war world to describe and classify the different socio- economic blocs on the planet.

I am not a mathematician but ‘tiers’ cannot be a fraction here as ‘Le tiers-monde’ doesn’t represent a third of the planet . It signifies the countries which are under-developed so it represents an order - 

Le premier monde,  being the richest capitalist  countries with a market economy,

Le deuxième monde were the socialist countries of the former Eastern block,

Le tiers monde which were the under-developed countries, i.e. poor countries .

This has changed and is changing all the time and the term has become out of favour as pejorative I presume...

People like to speak of ‘Les pays les moins avancés’ now as a kinder representation of those countries with very low economic status.

Hope this helps!