Definition
A skid is a turning error in which the airplane's tail swings to the outside of the turn because the rate of turn is too high for the bank angle being held. The airplane is yawing faster than the bank wants to carry it, producing sideways movement away from the center of the turn. It is felt as the pilot being pushed toward the outside of the turn and is corrected by either reducing rudder pressure or increasing the bank.
Plain English
A skid is when the airplane's tail slides out toward the outside of a turn because too much rudder is being used for the amount of bank. The airplane is turning too fast for how much it is leaning.
Context Anchor
Seen during ground reference maneuvers such as a rectangular course, and in any turn where rudder and bank are not balanced.
Derivation
From the everyday sense of "skid" -- to slide sideways across a surface, like a car skidding outward on a curve. The aviation meaning keeps that picture: the airplane is sliding sideways through the air toward the outside of the turn.
Why Pilots Care
A skidding turn at low altitude -- particularly the base-to-final turn -- is a classic setup for a stall/spin accident. Recognizing and correcting a skid early is a basic safety skill, not just a coordination issue.
Intuition Check
A skid in this context is not mainly about tires sliding on pavement. In flight, a skid means the airplane is sliding sideways through the air during a turn because the turn is not balanced.
Example Sentence 1
While flying the rectangular course, the student added too much rudder in the downwind-to-base turn and the airplane entered a skid.
Example Sentence 2
The pilot reduced rudder pressure to eliminate the skid and restore coordinated flight before continuing the pattern.