Plain English Meaning

The main wheel on the side the wind is coming from. In a crosswind landing, it's the wheel that touches the runway first.

Full Definition

During a crosswind landing, the main landing gear wheel on the side of the airplane that the wind is blowing from. In a properly executed crosswind touchdown, this wheel contacts the runway first while aileron is held into the wind, before the downwind main wheel and finally the nosewheel settle onto the surface.

Context Anchor

Encountered when learning crosswind landings, especially the touchdown step where one main wheel may touch before the other.

Derivation

Upwind' means toward the direction the wind is coming from — into the wind. So the upwind main wheel is simply the main wheel on the into-the-wind side of the airplane.