Plain English Meaning
The tightest circle the aircraft can make when taxiing with the steering turned all the way to one side.
Full Definition
The smallest radius an aircraft can pivot through on the ground when its nosewheel (or tailwheel) is deflected to its maximum steering angle. It defines the tightest ground turn the aircraft is physically capable of making and is used to determine whether the aircraft can negotiate taxiways, ramps, and turnarounds.
Context Anchor
Seen in discussions of aircraft performance, steep turns, maneuvering near the ground, and obstacle avoidance.
Derivation
Radius comes from a Latin word meaning a spoke or ray from the center of a circle. That helps because a turning aircraft is moving around part of a circle, and the is the distance from the center of that circle to the aircraft.