What is this data structure/concept where a plot of points defines a partition to a space
What you described is Voronoi diagram.
Here is an excerpt from Wikipedia.


In the simplest case, shown in the first picture, we are given a finite set of points ${p_1, \cdots, p_n}$ in the Euclidean plane. In this case each site $p_k$ is simply a point, and its corresponding Voronoi cell $R_k$ consists of every point in the Euclidean plane whose distance to $p_k$ is less than or equal to its distance to any other points. Each such cell is obtained from the intersection of half-spaces, and hence it is a convex polygon. The line segments of the Voronoi diagram are all the points in the plane that are equidistant to the two nearest sites. The Voronoi vertices (nodes) are the points equidistant to three (or more) sites.