Definition
A landing in which the airplane contacts the runway while still moving sideways relative to the runway centerline -- either because a sideways drift was not corrected before touchdown, or because the airplane was still crabbed into the wind with its longitudinal axis not aligned with the direction of travel. The result is that the landing gear touches down with a side load, because the wheels are forced to roll in a direction the airplane is not pointed.
Plain English
Landing while the airplane is still moving or pointed sideways compared to the runway. The wheels touch the ground while being dragged sideways, which puts a hard sideways force on the landing gear.
Context Anchor
Seen in crosswind landing training, especially during the flare and the last moment before the wheels touch the runway.
Derivation
Drift means the airplane is being pushed sideways by the wind across the runway. Crab is the angled, sideways-pointing posture used to track straight while the wind pushes from the side -- named after the crab's sideways walk. A touchdown in either condition means the sideways component was not removed before the wheels met the ground.
Why Pilots Care
Produces side loads on the landing gear that can cause loss of directional control, runway departure, or gear damage.
Intuition Check
Drift does not just mean being a little off course; here it means sideways motion at the moment the wheels touch. A crab is not always wrong during the approach, but touching down while still crabbed can create a control problem.
Example Sentence 1
The instructor pointed out that the gusty crosswind had caused a touchdown in a slight crab, and reminded the student to straighten the nose with rudder just before the wheels touched.
Example Sentence 2
A touchdown in a drift or crab on a narrow runway quickly leads to the airplane drifting toward the edge lights.