"Closure" is the missiological technical term for "finishing the task."
"Closure" can mean different things to different people, since the "finish line" is often defined differently. Some examples:
For our purposes, let's consider "closure" at a "minimum definition": everyone has access to the Gospel. (Other definitions require roughly the same kinds of work, albeit more of it.)
To achieve closure in any single people or place would mean everyone among the people or in the place has "crossed the finish line." For 10 people, this is fairly easy. For 100,000, it's far more difficult. To achieve closure for the entire world - across multiple languages, and in the midst of governmental restrictions - is so challenging that it hasn't been achieved in 2,000 years.
The most obvious method for achieving it would be to deliver the Gospel as a mass produced message, translated into every available language, within a short period of time. Yet part of the problem with such an effort is the very public nature of it.
An example of the difficulties of great, public, global campaigns is the campaign to eradicate polio. This has little (at least on the face of it) to do with spiritual issues, and even governments are generally in favor of it. Yet it has aroused fear of conspiracy, and vaccination workers have been killed.
A global campaign to present the gospel at a single point in time would obviously be fought against and blocked in many places. Realistically, it will have to be done bit by bit, place by place, people by people.
This kind of bit-by-bit progress encounters all the same issues as a global campaign, but it can slowly circumvent them over time. Governments that block the gospel during global campaigns today don't last forever. Walls come down, governments change, new platforms and technologies make the spread of the Gospel possible.
But since it must be done over time, closure encounters another problem that once-in-a-moment campaigns do not: demography.
In every place and among every people, new people are being born every day (and others are dying).
The closure achieved by delivering a message to the population of today misses the children born today. In three to five years, when those children are old enough to understand the message, it will have to be delivered to them. Today's message doesn't count for that moment.
If people hear the message today, but do not respond, they are still non-believers. They will not pass the message on to their children. Thus, the children of non-believers grow up unevangelized (even though the parents, hearing the message, were themselves once evangelized), unless they hear the message from an outside source.
If "Closure" is "Everyone has access to (or hears) the Gospel at least once in their lifetime," then this situation means each generation must hear the Gospel. Several of you are thinking, yes, this is obvious. But it's important: the primary reason peoples and places become unevangelized over time is the loss of gospel witness. If there is no local church, the gospel witness must come from the outside, and that is little different than a series of "localized campaigns" held over time, and faces much the same challenges as global campaigns.
"Sustainable closure" is my term which means the individuals in a people group or a place are continuously evangelized - the current generation, and future generations. The strategy simply calls for building up "sustainable closure" in each people and place, group by group, place by place, until all of them are sustainably closed. We don't worry about "losing" progress in one place as we go on to another because we leave the place in a position of sustainable closure.
The primary way this is done is to plant a church among a people or in a place that can achieve this. And that, by definition, is "reached": a people group possessing a church that can evangelize the group to its borders [here I would add, at least once per generation, and ideally once per year] without cross-cultural assistance.
(You can also apply the same definition to "place" provided that you say the church evangelizes all peoples within the place.)
"Sustainable closure" has the added benefit that, with a church that can reach everyone in a place, you are reaching all of the individuals, not just a minority.