Plain English Meaning
The wing on the far side from where the wind is coming from. Wind from the left means the right wing is the downwind wing.
Full Definition
The wing on the side of the airplane opposite the direction the wind is coming from during a crosswind takeoff or landing. If the wind is blowing from the left, the right wing is the downwind wing.
Context Anchor
Used during crosswind takeoffs and landings when describing which wing is upwind and which wing is downwind.
Derivation
Downwind' simply means 'in the direction the wind is going.' The downwind wing is the one on the side the wind is blowing toward — the side away from the wind source.
Why Pilots Care
Uncorrected rise of the downwind wing at can cause a or loss of .