Plain English Meaning
The fast-moving stream of air the propeller throws back behind itself. It hits the tail and rear of the airplane harder than the regular wind around the aircraft.
Full Definition
The column of air accelerated rearward by a spinning propeller. This air moves faster than the surrounding airflow and spirals around the fuselage, striking the tail and control surfaces with greater force than ambient wind.
Context Anchor
Encountered during engine start, run-up, and taxi, especially when power is added while the airplane is near people, loose objects, or other aircraft.
Derivation
Slipstream comes from the idea of air past or being pushed past an object. The propeller pulls air in and accelerates it rearward, creating a distinct stream of air that '' along the side of the aircraft faster than the surrounding air.