Plain English Meaning

It's the dangerous combination of a wing losing its lift and the aircraft then rolling and rotating downward. The wing quits flying first, and then the aircraft starts spinning toward the ground.

Full Definition

A stall/spin is an accident sequence in which an aircraft's wing exceeds its critical angle of attack (the stall) and, with uncoordinated flight at that moment, one wing drops and the aircraft enters an autorotating descent (the spin). The two events are paired because a spin cannot occur without a stall first, and stalls that happen with rudder or aileron misuse near the ground frequently develop into spins before recovery is possible.