Plain English Meaning
How fast the nose of the aircraft is swinging left or right around a vertical line through the aircraft, measured in degrees per second.
Full Definition
The speed at which an aircraft is rotating about its vertical axis, expressed in degrees per second. The turn-and-slip indicator senses this rate gyroscopically and displays it as a needle deflection, which under coordinated flight corresponds to the rate of turn.
Context Anchor
Seen in discussions of the turn-and-slip indicator, which shows how fast the aircraft is turning.
Derivation
Yaw originally described the unsteady side-to-side motion of a ship straying off course. Applied to aircraft, it describes the same kind of motion -- the nose swinging left or right -- and 'rate of yaw' is simply how quickly that swing is happening.