The term "unreached" was made popular by mission leaders seeking to prioritize ethnic people groups with little or no exposure to the Gospel. Today, the term has become a catchword used by all kinds of ministries that do not necessarily focus on unreached people groups.
Ignorance is bliss but it can also be deadly. One of the most dangerous misuses of Christian phrases I’ve heard recently is the overuse of the phrase, “unreached people group.” I’ve heard people use that phrase to talk about efforts to reach college campuses, friends, clubs, and neighborhoods. It is a term that has been used more and more in the Christian subculture to validate missional status quo rather than raise the alarm for people groups who are entirely cut off from the gospel.
When God gives you a task, it is infinitely important that you know the definition of the task. God has promised to reach all the nations of the earth and commissioned us as His ambassadors in that work. In order to be good stewards of this mission we must have a firm grip on the extent of the task, which, in the work of world missions, brings us to look closely at the terms that the Bible uses for the task. Namely, what does it mean to reach the nations?
If the command given by Jesus is to make disciples of all nations (or ethnic groups), then common sense would tell us our job is to find those nations (ethnic groups) that have not been discipled (taught to be followers of Christ). The vast majority of these unreached people live in an area of the world nicknamed the "10/40 window."