According to this site:
The “10/40 Window” is a missions term used to refer to the rectangular area of the world from 10 degrees to 40 degrees latitude. This rectangle stretches from Southeast Asia, across the Middle East, over to West Africa. The 10/40 Window is home to approximately 97% of the 2.5 billion unreached people in the world today. There are over 10,000 people groups in this window; 6,000 of those have never had access to the gospel. Think about this… Every day, more than 70,000 people die in this world without having ever heard the gospel of Jesus Christ.