Plain English Meaning
How fast a point on the spinning propeller blade is moving in a circle. The tip of the blade moves faster than the part near the center, because it has farther to go in one turn.
Full Definition
The speed at which a point on a rotating propeller blade moves around the hub, expressed in distance per unit of time. Because the blade is spinning, points farther from the hub travel a greater distance per revolution and therefore have a higher rotational velocity than points closer to the hub, even though the whole blade turns at the same RPM.
Context Anchor
Seen in propeller theory when comparing the blade’s spinning motion with the airplane’s forward motion through the air.
Derivation
Rotational comes from the Latin rotare, meaning 'to turn.' Velocity comes from the Latin velocitas, meaning 'speed.' Together: the speed of turning. In propeller theory, it specifically means the speed of a point traveling around the hub.