Plain English Meaning
A small ball in a curved 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 the ball slides to one side, the airplane is sliding through the turn the wrong way and the pilot needs to correct with the rudder.
Full Definition
A flight instrument component, typically a small ball in a curved liquid-filled tube mounted below the turn indicator or turn coordinator, that shows whether the aircraft is in coordinated flight. When the ball is centered, the turn is coordinated. When the ball is displaced toward the inside of the turn, the aircraft is slipping; when displaced toward the outside, the aircraft is skidding.
Context Anchor
Seen on instruments such as a turn coordinator or turn-and-slip indicator during , , and instrument .