Plain English Meaning
A controlled, deliberate sideways slide through the air, made by banking one way and pressing the rudder the other way. It steepens the descent and lets the pilot lose altitude quickly without speeding up, or keep the nose pointed straight down the runway in a crosswind landing.
Full Definition
A flight maneuver in which the aircraft is banked toward one side while opposite rudder is applied, causing the airplane to descend along a flight path that is not aligned with its longitudinal axis. The result is increased drag and a steeper descent without an increase in airspeed, and a sideways component of motion through the air.
Context Anchor
Pilots encounter slips during crosswind landings, when losing altitude without gaining much speed, and when checking whether the airplane is flying in a coordinated way.
Derivation
From Old English 'slipan,' meaning to slide or glide. The aircraft is literally sliding sideways through the air rather than flying straight along its nose direction.
Analogy
A slip is like walking forward while your body is angled slightly sideways. You are still moving ahead, but not cleanly in the direction you are facing.
Intuition Check
Slip does not mean the airplane has lost traction, like a car on ice. In flight, a slip means the airplane is moving sideways through the air because its nose and flight path are not aligned.
Example Sentence 1
Coming in high over the trees, the pilot used a forward slip to lose altitude without picking up extra speed.
Example Sentence 2
The pilot used rudder and opposite aileron to slip the airplane sideways during the approach.