Fitts's law
Fitts's law relates the time needed to reach a target to its distance and size
What it means, how it is used and where it comes up, with its related concepts.
What it is and how it works
In interfaces it helps to design controls that are easier to select, especially for frequent actions. Its application should consider the device and people's abilities. Enlarging a useful target and separating it from incompatible actions can reduce interaction errors.