Fitts's law
Also said as Fitts law, Target size rule
The time it takes to reach a target grows with distance and shrinks with size, in a precise way. Paul Fitts measured it in 1954: time = a + b × log2(2D ÷ W), so doubling the distance or halving the width adds the same fixed amount.
It tells you where to spend screen space. Common actions should be big and close, and anything rare can be small and far away. Screen edges and corners are effectively huge targets for a mouse, as the pointer stops there.
Say it
“Make the buy button at least 80 px tall and put it where the thumb already rests. Leave anything rare in a small corner icon.”
Where you’ve seen it
Radial menus such as Mass Effect's power wheel are Fitts's law at work: all the choices sit next to the centre, equally close, and each one is a wide wedge that's hard to miss with a thumbstick.
Not to be confused with friction
Fitts's law is about the time for one move to one target. Friction is the whole cost of a task, including how many of those moves it takes.