Definition
The transition from final approach descent to the level landing attitude during a crosswind landing, in which the pilot smoothly raises the nose to slow the descent while continuing to hold the airplane aligned with the runway centerline using rudder, and counters drift with aileron into the wind. Throughout the flare, aileron pressure into the wind is increased as airspeed bleeds off, because the slower the airplane flies the more control deflection is needed to produce the same correction.
Plain English
It is the moment just before touchdown in a crosswind landing when the pilot eases the nose up to stop the descent, keeps the nose pointed straight down the runway with rudder, and uses aileron pressure toward the wind to stop the airplane from drifting sideways. As the airplane slows, the pilot must keep adding more aileron into the wind.
Context Anchor
You encounter this during the last seconds of a crosswind landing, after the final approach and just before touchdown.
Derivation
"Round out" describes the rounding of the flight path from a descending line into a level path just above the runway. "Flare" comes from the same word used for a flame spreading outward, meaning to widen or open up — here, the flight path opens out from a descent into level flight.
Why Pilots Care
Improper round out in a crosswind allows the aircraft to drift, imposing side loads on the landing gear or causing loss of directional control at touchdown.
Intuition Check
Do not think of the flare as simply pulling the nose up and waiting to land. In a crosswind round out, you must keep steering and correcting for the side wind until touchdown.
Example Sentence 1
During the crosswind round out, the student kept the nose aligned with the centerline with right rudder while adding left aileron to stop the drift.
Example Sentence 2
A well-timed crosswind round out allows the main wheels to settle onto the centerline with no sideways movement.