Plain English Meaning
A little ball in a curved glass tube on the instrument panel that tells the pilot whether the airplane is turning smoothly. If the ball sits in the middle, the turn is balanced. If it slides to one side, the airplane is sliding sideways through the air and the pilot needs to correct it with rudder.
Full Definition
The small ball, located inside a curved tube at the bottom of the turn coordinator (or turn-and-slip indicator), that shows whether the airplane is in coordinated flight. When the ball is centered, the airplane is coordinated. When the ball is displaced to one side, it indicates a slip or a skid, depending on the relationship between the bank and the rate of turn.