Plain English Meaning
Two spinning tubes of air left behind by an aircraft's wings, one off each wingtip, spinning in opposite directions. They are the main hazard in another aircraft's wake.
Full Definition
A pair of spinning air masses trailing from an aircraft's wingtips that rotate in opposite directions to each other. The vortex from the left wingtip rotates clockwise (viewed from behind), and the vortex from the right wingtip rotates counterclockwise. They form whenever a wing is producing lift and persist behind the aircraft as wake turbulence.