Definition
A reading on the turn-and-slip indicator showing that the aircraft is slipping toward the inside of a turn, where the rate of turn is too slow for the angle of bank. The needle deflects toward the direction of turn while the ball rolls to the inside (low) side of the turn, indicating uncoordinated flight that requires more rudder in the direction of the turn.
Plain English
The instrument is telling you the airplane is sliding sideways into the turn because you are banked more than the turn rate calls for. The ball has rolled down to the inside of the turn, and you need to add rudder on that same side to straighten things out.
Context Anchor
Seen on the ball portion of a turn-and-slip indicator while checking whether a turn is coordinated.
Derivation
Slip comes from the Old English slipan, meaning to slide. The aircraft is literally sliding sideways through the air rather than turning cleanly around its vertical axis.
Why Pilots Care
An uncorrected slip wastes energy, produces inaccurate heading and airspeed indications, and can mask other instrument errors.
Intuition Check
Do not read “slip” here as a tire or wheel losing grip. In this instrument context, a slip means the airplane is moving sideways through the air during a turn, and the ball shows that by moving off center toward the inside of the turn.
Example Sentence 1
During the steep turn to the left, the ball rolled left and the needle showed a slip indication, so the pilot added left rudder to recenter the ball.
Example Sentence 2
After entering the holding pattern, the slip indication appeared because the pilot had not coordinated the turn with rudder.