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.
Plain English
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.
Context Anchor
Seen on instruments such as a turn coordinator or turn-and-slip indicator during instrument flying, turns, and basic attitude instrument work.
Derivation
Slip and skid describe two different ways an aircraft can move sideways through a turn. Slip comes from the old sense of sliding inward; skid comes from the sense of sliding outward, like a car skidding wide on a corner. The instrument indicates which of these is happening, hence slip-skid indicator.
Why Pilots Care
Keeping the indicator centered maintains coordinated flight, improves control effectiveness, reduces drag, and lowers the risk of unintended stalls or spins.
Analogy
It is like watching a marble in a curved track: if the marble stays in the middle, the motion is balanced; if it rolls to one side, something is out of balance.
Intuition Check
Do not read slip and skid here as casual words for losing traction on the ground. In flight, they mean the airplane is not moving cleanly through the air during a turn.
Example Sentence 1
During the turn to final, she glanced at the slip-skid indicator and added a touch of right rudder to center the ball.
Example Sentence 2
When entering a climbing turn, the student watched the slip-skid indicator to avoid a skid that would increase stall speed.