Plain English Meaning
An instrument with a needle and a ball. The needle tells you how fast you are turning. The ball tells you whether your feet and hands are working together properly during the turn.
Full Definition
A flight instrument that shows the rate at which the airplane is turning (yawing) and whether the turn is coordinated. It uses a gyroscope to drive a needle that deflects left or right to indicate turn rate, and a curved tube containing a ball in liquid to show slip or skid. When the ball is centered, the turn is coordinated; when the ball is off-center, the airplane is slipping or skidding through the turn.
Context Anchor
Seen on the , especially during and when checking whether a turn is coordinated.