Relative wind is the angle of airflow as it impacts an airfoil.
Movement of an airfoil through the air creates relative wind. The direction of airfoil in relation to the air changes the angle of the airflow, or relative wind. Relative wind is parallel but in the opposite direction of the airfoil’s direction.
With fixed-wing aircraft, this is a simple concept. With rotary wing aircraft, there are several other factors impacting the relative wind, mainly rotation of the rotor blades, and the induced flow from lift production. With helicopters, when someone refers to relative wind, they are usually referring to resultant relative wind.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-8
Principles of Helicopter Flight, 2nd Edition, pg. 47
FM 3-04.203-2007 Fundamentals of Flight pg. 1-8