Plain English Meaning
It is the small ball you see at the bottom of the turn coordinator or turn-and-slip indicator. If the ball is in the middle, the turn is smooth and balanced. If the ball slides off to one side, you are skidding or slipping and need to adjust the rudder.
Full Definition
A flight instrument component, usually a curved glass tube containing a ball in fluid, that shows whether the airplane is in coordinated flight by indicating sideways (lateral) acceleration. The ball is centered when the forces of the turn are balanced; it moves toward the inside or outside of the turn when the rudder and bank are not coordinated.