Definition
The wing on the side of the airplane facing into the wind during a crosswind takeoff or landing. In a crosswind, this is the wing the wind is striking first, and it tends to be lifted by the relative wind unless the pilot deliberately holds it down with aileron input.
Plain English
The wing pointing toward where the wind is coming from. If the wind is blowing from your left, your left wing is the upwind wing.
Context Anchor
Used during taxi, takeoff roll, landing roll, and other ground operations when wind is coming from the side.
Derivation
Upwind means 'toward the source of the wind' — the direction the wind is coming from. The upwind wing is simply the wing on that side of the airplane.
Why Pilots Care
Failure to hold the upwind wing down allows it to rise, creating uneven lift that can yaw the airplane downwind and cause loss of directional control on the runway.
Intuition Check
Do not think of “upwind” as the direction the wind is moving. In aviation use here, the upwind wing is on the side the wind is coming from.
Example Sentence 1
With the wind from the left, the pilot held full left aileron during the takeoff roll to keep the upwind wing down.
Example Sentence 2
Once airborne the pilot gradually neutralized aileron as both wings began to fly equally.