An unreached people group, as defined by the Joshua Project, is "a people group among which there is no indigenous community of believing Christians with adequate numbers and resources to evangelize this people group without outside assistance."
Note: In missiological literature, you will see these groups sometimes referred to by the acronym UPG.
-- Howard Culbertson,
Biblical passages on world evangelism Audio of Matthew 28:18-20 Historic missionary slogans Brief lecture on the Biblical foundations for missions Among all nations with power: Four reflections What is the fate of the unevangelized? Answers to an oft-asked question Missionary statistics -- including world population and number of languages Apple pickers: deploying resources to evangelize the unreached List of "gospel-ready" countries