Plain English Meaning

The fastest speed at which you can make a sudden, full control input in one direction without risking damage to the airplane. Below this speed, the wing will stall before the airframe is overstressed.

Full Definition

The maximum speed at which application of full or abrupt flight control inputs about a single axis will not cause structural damage to the airplane, expressed at the airplane's current operating weight. VO is established by the manufacturer for airplanes certificated under newer airworthiness standards and is generally lower than VA (design maneuvering speed). At or below VO, the airplane will stall before flight loads can exceed its structural limits.