Plain English Meaning

The aileron on the side the wind is coming from. While taxiing in a crosswind, the pilot turns the control wheel into the wind so this aileron points up, helping hold the wing down so the wind cannot lift it.

Full Definition

The aileron on the wing facing into the wind, deflected upward by the pilot during taxi in a crosswind to keep that wing from being lifted by the wind. The control wheel or stick is held toward the wind, which raises the upwind aileron and lowers the downwind aileron.

Context Anchor

Seen in taxi instructions for holding the flight controls correctly when wind is blowing across or partly across the airplane.