Plain English Meaning

The ball has slid to the outside of the turn. This means you are turning too fast for how steeply the wings are banked. The fix is to add more bank or reduce the rate of turn until the ball returns to the center.

Full Definition

On a turn-and-slip indicator, a skid indication is shown when the ball rolls to the outside of the turn, indicating that the rate of turn is too high for the angle of bank — the aircraft is being pushed sideways toward the outside of the turn due to insufficient bank for the rate of yaw being applied.

Context Anchor

Seen on the turn-and-slip indicator, also called the needle and ball, during instrument turns.

Derivation

Skid comes from an old word meaning to slide sideways without , like a car outward on a curve. The aircraft is doing the same thing — sliding toward the outside of the turn instead of carving cleanly through it.