Ball
From FlightSim
The ball in the inclinometer (often part of the turn and bank or turn coordinator instruments) sits in a small curved tube. and is free to move as sideways forces on the aircraft change. In the center of this small tube are a pair of small vertical lines which are just the width of the ball apart.
When the ball is centered (evenly between the two lines) there are no side forces on the aircraft, meaning that the aircraft is properly aligned with the relative wind. This results in the least drag on the aircraft. If the ball is not centered, the pilot applies rudder pressure on the side to which the ball is displaced until the ball is again centered.
See also the turn coordinator page or the slip indicator page on Wikipedia.
