Fitts' Law
Fitts' Law is a scientific model that predicts the speed of the act of pointing, whether it be with a finger or with a point device such as a mouse. Specifically, it states that the time to move from where a person is pointing to a target location is a function of the ratio of the distance to the target and the target's width.
While Fitts' Law was developed in the mid-1950's, it is used significantly in modern UI/UX design. In this application, it encourages neutral placement and shorter distances between commonly used elements as well as appropriate sizing of said elements.