Definition
The region of altered airflow close to the ground -- generally within one wingspan of the surface -- where the proximity of the ground reduces induced drag and changes the lift and control characteristics of an airplane in flight.
Plain English
The pocket of air close to the ground, roughly within one wingspan of the surface, where the airplane behaves differently because the ground is interfering with the airflow under the wings.
Context Anchor
You encounter this term during takeoff and landing discussions, especially when learning how the airplane behaves just after liftoff or just before touchdown.
Derivation
Ground effect' refers to the effect the ground has on the airflow around the wing. 'Area' here marks out the physical zone -- close to the surface -- where that effect is strong enough to matter.
Why Pilots Care
Pilots must anticipate the sudden performance change that occurs when entering or leaving this zone, or they may over-rotate on takeoff or float excessively on landing.
Grounding Statement
Picture the airplane lifting off and climbing through an invisible cushion of air about one wingspan thick -- inside that cushion the wing works more efficiently; once the airplane climbs above it, the cushion is gone.
Intuition Check
Do not think of the ground effect area as a marked place on the airport surface. It is the low space above the surface where the airplane’s wings are affected by being close to the ground.
Example Sentence 1
On a hot day at a high-elevation airport, the airplane lifted off but struggled to climb out of the ground effect area until it accelerated to a proper climb speed.
Example Sentence 2
On a short-field takeoff the extra lift available inside the ground effect area allowed the airplane to become airborne at a lower speed than normal.