The 10/40 Window is a term coined by Christian missionary strategist Luis Bush in 1990 to refer to those regions of the eastern hemisphere, plus the European and African part of the western hemisphere, located between 10 and 40 degrees north of the equator, a general area that in 1990 was purported to have the highest level of socioeconomic challenges and least access to the Christian message and Christian resources on the planet.
The 10/40 Window concept highlights these three elements: an area of the world with great poverty and low quality of life, combined with lack of access to Christian resources. The Window forms a band encompassing Saharan and Northern Africa, as well as almost all of Asia (West Asia, Central Asia, South Asia, East Asia and much of Southeast Asia). Roughly two-thirds of the world population lives in the 10/40 Window. The 10/40 Window is populated by people who are predominantly Muslim, Hindu, Buddhist, animist, Jewish or atheist. Many governments in the 10/40 Window are formally or informally opposed to Christian work of any kind within their borders. 
This region of the world was previously known to Christians as the "resistant belt," as noted by Luis Bush at the 1989 Lausanne II Conference in Manila. In 1990, Luis' research led to a meeting with the developer of the first PC-based GIS software. They analyzed the region (see below) using a box between 10 and 40 degrees north latitude and called it the 10/40 Box. A few weeks later, Luis' wife Doris recommended renaming it the 10/40 Window, stating that this region ought to be seen as a "window of opportunity." The analysis and concept was a generalization that focuses on a region, not a sharp boundary defining what is a priority, and what is not. For this reason, many missiologists prefer to use the phrase "10/40 Window Region."
Before being called the "resistant belt," the Islamic portions of this region, as well as selected unreached Buddhist and Hindu areas, were referred to as the "unoccupied fields" by Samuel Zwemer, in his book by that same title, published in 1911.
Some[who?] have objected to such a broad-brush term which seems to imply a unifying characteristic of the window when in fact no large area of the planet is completely homogenous in cultural attributes.
that part of the world did have least access to Christian resources. Note the emphasis on access not percent Christian. The West has ubiquitous access to such resources; this area of the world did and does not.
This research deals in overall population characteristics. The 10/40 Window is a term that helps people visualize the general area of the analysis, where the above characteristics are generally true, but with exceptions proving it is only a generalization. Some examples of the exceptions:
The Window article refers to the "poorest of the poor" living in that region (based on late-1980's per-capita GNP under US$500). Of the three billion people living in such poverty-stricken nations, 82% lived in the 10/40 Window. Yet such a result contrasts with 10/40 Window nations such as South Korea and Japan. Japan boasts the world's third largest economy, while South Korea the eleventh. Such nations were strong throughout the late 20th century.
Geographically, the Window includes the Philippines and Portugal, which are both nations with a Roman Catholic Christian majority, also Greece where almost 98% of the population belongs to the Greek Orthodox Church, plus South Korea, which is home to the largest single congregation church in the world and is also the second largest missionary sending nation in the world. The window does not encompass Indonesia or Sri Lanka, nations with very little access to Christian resources.
Gaining widespread use
Over the years, the 10/40 Window has evolved from a specialist term used by Christian missiologists to assumed vocabulary for Christians in the West. It is an emerging term in the secular press and can be found in press style glossaries. Non-western writers and organizations also refer to the 10/40 Window. In addition, those opposed to the idea of evangelism make use of the term.
The original 1990 GIS 10/40 Window analysis produced several insights, among them showing that the nations of the 10/40 Window represent (as of the research date):
82% of the poorest of the world's poor (per capita GNP less than US$500 per year),
84% of those with lowest quality of life (life expectancy, infant mortality, and literacy),
the hub of the world's major non-Christian religions (Islam, Buddhism, Hinduism, etc.,)
close to 100% of those who are both most-poor and have least-access to Christian resources (two-dimensional analysis)
The least Christian resource investment and least sharing of the Christian message
Non-Christians in the 10/40 Window by religion
The first edition GIS analysis maps highlighted the three major religious blocks in the 10/40 Window, specifically the majority Muslim, Hindu and Buddhist nations. Population estimates at the time for the year 2000 (from Operation World) were given as:
28 Muslim Countries, 1.1 Billion population est (2000)
2 Hindu Countries, 1.1 Billion population est (2000)
8 Buddhist Countries, 237 Million population est (2000)
Later updates have been based more on census data and other estimates rather than forward-looking population estimates. The cited reference provides the following estimate of "unreached" non-Christian populations in the 10/40 Window:
865 million Muslims
550 million Hindus
275 million Buddhists
140 million in 2550 tribal groups (mainly animist)
^"The 10/40 Window". Time Magazine. June 30, 2003. Archived from the original on April 28, 2007. Retrieved March 1, 2007.. The link is to the map, which is part of an extensive cover story.
^ abcdeWorld FactBook., edition available in 1990. At the time, the authoritative source for socioeconomic and political metrics for every nation. Extreme poverty was denoted as under US$500 per capita GNP (in 1990 dollars). Human suffering was measured by the Quality of Life index, precursor to today's Human Development Index
^See the Analysis section of this article for research-based details and cited references.
^"Access" is generally defined using a variety of metrics. What is least controversial is those areas with least access throughout history, as all metrics for such areas are zero or close to zero. Examples of metrics used include the presence of (Christian) work and workers (of any kind, whether community development, health, business, child care, house servants, etc), media in an appropriate language (print, TV, radio, web, etc).,
^ abcdBarrett, David B.; Kurian, George T.; Johnson, Todd M. (eds.) (2001). World Christian Encyclopedia (2nd ed.). Oxford University Press. pp. 1739pp. ISBN0-19-507963-9. Archived from the original on February 6, 2003. Much of its data is available online at the ("World Christian Database". Brill. Archived from the original on March 4, 2007. Retrieved March 1, 2007.) Mind-numbing in its details (with some areas of unique value), but the introduction and definitions in the paper edition are quite helpful to understanding.
^"otherwise". Web.archive.org. February 15, 2008. Archived from the original on 2008-02-15. Retrieved November 20, 2011.
^AD2000. Three Religious Blocks & The 10/40 Window (Map). Cartography by GMI/GRDB (August 1, 1990 ed.).
^AD2000. Islam & & The 10/40 Window (Map). Cartography by GMI/GRDB (August 1, 1990 ed.).
^AD2000. The Poor, The Unevangelized, & The 10/40 Window (Map). Cartography by GMI/GRDB (August 1, 1990 ed.).
^Well below 10 percent in any of the study populations. Each of the cited maps provides side data on population, Christian involvement, etc in the various study populations (poor, low quality of life, non-Christian, etc)
^AD2000. The 55 Least Evangelized Countries & The 10/40 Window (Map). Cartography by GMI/GRDB (August 1, 1990 ed.).
^Note that this number is higher than the census-based world total of 14 million. However, even authoritative Jewish sources state that many Jews do not identify themselves in population censuses.