Definition
An uncoordinated flight condition in which the airplane's lateral motion is not aligned with its longitudinal axis. In a slip, the airplane is banked more than the rate of turn requires, causing it to slide toward the inside of the turn. In a skid, the airplane is banked less than the rate of turn requires, causing it to slide toward the outside of the turn. The slip/skid indicator (the ball in the turn coordinator or inclinometer) shows this condition: the ball moves toward the slip side or the skid side of center.
Plain English
Slip and skid are two ways an airplane can be out of balance in a turn. In a slip, the airplane is leaning into the turn too much and slides inward. In a skid, it isn't leaning enough and slides outward. The little ball on the instrument panel shows which one is happening.
Context Anchor
Seen during taxi checks and in flight when checking the ball in the turn coordinator or similar instrument.
Derivation
Slip comes from the Old English slipan, meaning to slide smoothly. Skid comes from a Norse word for a piece of wood placed under something to make it slide sideways. Both words describe sideways sliding motion, which is exactly what the airplane is doing when it is uncoordinated.
Why Pilots Care
Uncoordinated flight reduces control effectiveness and increases stall risk in certain conditions.
Grounding Statement
If the ball is not centered in flight, the aircraft is being pushed sideways instead of moving cleanly through the turn.
Intuition Check
Do not read slip/skid as just a tire problem. Here it means sideways aircraft motion or the instrument indication of that motion.
Example Sentence 1
During the climbing left turn after takeoff, the ball drifted to the right, showing a skid, so the pilot added right rudder to center it.
Example Sentence 2
A persistent skid on final approach can lead to an unstable approach.