Plain English Meaning
The twisting, downward spiral an airplane follows when it is in a spin -- rotating around itself while falling in a tight, screw-shaped path toward the ground.
Full Definition
The downward, spiraling flightpath traced by an airplane during a spin, in which the aircraft simultaneously rotates about its vertical axis and descends along a tight helical track around an imaginary vertical line.
Context Anchor
Used in spin awareness to describe the path an airplane may follow after a stall develops into a spin.
Derivation
From 'corkscrew,' the spiral metal tool used to pull corks from bottles. The shape of that tool -- a tight, descending spiral around a central axis -- closely matches the path an airplane traces during a spin, which is why the term was borrowed.