Definition:
Fitts Law often called as Fitts's Law. Fitts Law states that the time it takes to hit the target is a function of the size of the target and the distance to that target.
Figure: Fitts Law
Figure: Applying fitts law to mobile interface design
Figure: Mobile Dail Screen follows Fitts Law
And some more examples:
Figure: Fitts Law applicable
Figure: Fitts law applicable on volume buttons
Figure: Size of the target to the distane of the target