Timothy Tennent takes a few pages to discuss the merits of each term in his excellent book, Theology in the Context of Global Christianity. On the designation "Third World" (coined in 1952 by the French demographer Alfred Sauvy), Tennent says:
It quickly entered English as a helpful phrase to speak collectively about Africa, Asia, and Latin America. It was originally articulated in the context of the Cold War, when the world was polarized by the two "worlds" of the capitalistic, industrialized West and the communist, state-controlled East. It began as a political expression. Later, in the 1960s and 1970s, the expression "Third World country" began to be used in an economic
sense to refer to underdeveloped countries marked by poverty and disease.
With the collapse of the Soviet Union and the emergence of economic prosperity in places like Singapore and the oil-rich Middle East, the phrase has become increasingly dated, and this book does not employ the phrase."
Though Tennent doesn't employ the term "Third World," I continue to hear it often, and heard it again this morning on television.
On the term "Two-Thirds world," Tennent writes:
Later, the phrase "Two-Thirds World" emerged to counteract what was incorrectly perceived to be the statistical origin of the expression "Third World." This expression emphasized the sheer geographic size of Latin America, Africa, and Asia, though sometimes refers to the demographic size of the church in these continents.
What about the term "Global South?" Tennent writes
"The words "north" and "south" or "northern Christianity" and "southern Christianity" are also used regularly in contemporary missions literature. . . Latin America, Africa, and Asia are often referred to as "the South" or the "Global South," and Christians from North America and Europe are referred to as "the North." These terms are helpful, but they are difficult to apply without some qualifications now that we are seeing such a rapid expansion of the church in China, which it is difficult to refer to as the "South."
Tennent opts for "Majority World," as the best terminology to employ.
The "Two-Thirds World" is helpful, but it has been superseded more recently by the simpler phrase "Majority World." The 2004 Lausanne Forum for World Evangelization which I attended in Pattaya, Thailand, dedicated an entire working group to the theme, "The Two-Thirds World Church." It included participants from across the world, and one of their formal actions was to vote unanimously that the phrase "Majority World Church" be used. This is the best phrase currently available. It is to be preferred because it is simpler and less confusing to students just entering tghe discussion for the first time, and it helps to highlight the basic point that Africa, Asia, and Latin America are where the majority of the world's Christians are now located. (pp. xix, xx)