Plain English Meaning
How powerful the spinning air left behind an aircraft is. A heavy airliner flying slowly with its flaps up leaves behind very strong spinning air; a small, fast aircraft with flaps down leaves much weaker spinning air.
Full Definition
The intensity of the rotating air masses (wingtip vortices) trailing behind an aircraft in flight, determined primarily by the generating aircraft's weight, speed, and wing shape. Vortex strength increases as weight increases, as speed decreases, and when the wing is in a clean configuration (flaps and gear retracted). The heaviest, slowest, cleanest aircraft produces the strongest vortices.