An inverse square law is a scientific law (i.e., well-established model) asserting that some value depends upon the distance from something, more specifically on the reciprocal of the square of that distance (1/d²). If that something is a point or spherically symmetric body, then given any distance (i.e., the radius of a sphere centered on the point), the sum (integral) of the value across all points at any such distance is the same. An inverse square law generally implies three-dimensional Euclidean space, and serves as an approximation if the space is close to that, such as are current models of the universe.
Some common inverse-square examples:
Phenomena that are not inverse square: