Plain English Meaning
A spin happens when the wings stop flying and the airplane starts rotating downward in a tight spiral, nose-low, while falling fast. Both wings are stalled, but one is stalled more than the other, which is what makes the airplane rotate.
Full Definition
An aggravated stall in which the airplane descends in a corkscrew path while rotating about its vertical axis, with one wing more deeply stalled than the other. The airplane is simultaneously stalled, yawing, rolling, and pitching nose-down at a high rate of descent.
Context Anchor
Seen in discussions of stalls, at slow , and why pilots avoid letting the airplane get too slow while it is turning.