Plain English Meaning
How fast the corkscrew of air coming off the propeller is moving backward over the airplane.
Full Definition
The speed of the spiraling tube of air pushed rearward by a propeller. Slipstream velocity increases with engine power and decreases as the airplane's forward speed increases.
Context Anchor
Seen in takeoff, climb, and slow-flight discussions where propeller airflow affects lift, cooling, and control response.
Derivation
Slipstream comes from 'slip' (to slide past) plus 'stream' (a flow). It's the stream of air that slides past the airplane after being accelerated by the propeller. Knowing this helps because the term is literally describing a flow of air sliding rearward along the fuselage.