Definition
A deliberate flight maneuver in which the pilot cross-controls the airplane — applying aileron in one direction and opposite rudder — to fly the aircraft slightly sideways through the air. This increases drag and steepens the descent path without increasing airspeed, and is used to lose altitude or to compensate for a crosswind on landing.
Plain English
A controlled way of making the airplane descend faster, or stay lined up with the runway in a crosswind, by deliberately flying it slightly sideways using opposite rudder and aileron at the same time.
Context Anchor
Seen during approach and landing practice, especially when discussing forward slips, side slips, steep descents, and crosswind landings.
Derivation
The word slip comes from the Old English slipan, meaning to slide or move smoothly sideways. The airplane is, in effect, sliding partly sideways through the air rather than moving cleanly nose-first.
Why Pilots Care
Enables a steeper descent angle on short or obstructed runways without exceeding desired airspeed.
Intuition Check
Do not read slip here as a mistake or a loss of control. An intentional slip is a planned, controlled maneuver; the pilot chooses it and controls it.
Example Sentence 1
On final approach the pilot realized she was high, so she used an intentional slip to lose altitude without picking up speed.
Example Sentence 2
During a strong crosswind landing, the pilot used an intentional slip to keep the airplane aligned with the runway.