This post covers this concept and may be updated from time to time to improve it for clarity and answer questions.
We use terms to quantify the "remaining task" - words like "unreached," "unevangelized," "unengaged," and "least-reached."
Often we use these words interchangeably. This can be quite dangerous, as they do not mean the same thing, and we may not be meaning the same thing when we use them. An in-depth look at the history of these terms and how they have been used is given in the book World Christian Trends. Here, I'm just going to briefly summarize how they are used today.
"Unreached" is a term that was originally defined in a meeting of missiologists held in Chicago shortly after the whole idea of unreached peoples really took off. It was defined as, "a people group lacking a church that can evangelize the group to its borders without cross-cultural assistance."
"Unevangelized" as it is generally used was defined in the World Christian Encyclopedia (and World Christian Trends, its related work) as a mathematical equation for estimating the number of people within a people group that would have access to the Gospel at least once in their life time.
"Unevangelized" is used by others with slightly different definitions, so it's important to understand what is meant by "unevangelized" whenever it is used. Usually, when people refer to "2.3 billion unevangelized," they are referring to the number in the Status of Global Mission, which comes out of the World Christian Encyclopedia research.
"Unengaged" was created by Finishing the Task and is defined as a people group lacking a team with a church planting strategy. If a group of several million has a team of 2 or 3 that has "engaged it" with a church planting strategy, then it is "engaged" (but it may be "underserved" based on a ratio of 1 team per 50,000). The unengaged list is maintained by Finishing the Task and is a derivative of other lists.
"Least-reached" is a generic term used to refer to the core of the remaining task. It does not have a specific definition, and is often used when no specific definition is desired. The number of people considered "unreached" and those considered "unevangelized" are often different, because the definitions are different: