Definition
The component of the relative wind that flows opposite the airplane's flight path, striking the wing from the front. It is the airflow the wing actually 'sees' as it moves forward through the air, regardless of any natural wind blowing across the ground.
Plain English
The wind the wing feels coming at it from the front because the airplane is moving forward. It is created by the airplane's own motion through the air, not by weather.
Context Anchor
Seen in angle-of-attack discussions, especially when explaining the direction of airflow over the wing.
Derivation
Relative' comes from Latin relatus, meaning 'carried back' or 'referred to something else.' Here it means the wind is described relative to the airplane, not relative to the ground. 'Headwind' simply means wind coming from the front. So 'relative headwind' is the front-on airflow as experienced by the airplane itself.
Why Pilots Care
It directly changes airspeed over the wings and therefore changes the lift and drag produced at any given angle of attack.
Analogy
Stick your hand out of a car window on a still day. There is no weather wind, but you feel air pushing against your hand because the car is moving. That airflow is the 'relative wind' for your hand.
Grounding Statement
If the airplane moves through the air, the airplane feels airflow coming from the opposite direction of its motion.
Intuition Check
Do not read relative headwind as only a weather headwind. In this context, it means the oncoming airflow relative to the airplane’s movement through the air.
Example Sentence 1
As the airplane accelerates down the runway, the relative headwind over the wings increases until enough lift is produced for takeoff.
Example Sentence 2
When the relative headwind increased on final approach, the airplane needed a slightly lower pitch attitude to stay on the desired glide path.