The negative sign reflects the dominance of the nearest neighbors, and shows that work would have to be done to take the crystal apart into ions.
The crystal structure has each atom surrounded by eight nearest neighbors which lie at the corners of a cube.
"World missions" is perhaps a better term, for that includes our nearest neighbors, who are part of the world to which God has called us. And we are already there!
Solar flare forecasting method of combining support vector machine with nearest neighbors
Short-term traffic flow forecasting based on K-nearest neighbors non-parametric regression
