Plain English Meaning
How well a pilot can see at night. It works differently from daytime vision -- it takes time to develop, things are seen more clearly out of the corner of the eye than dead ahead, and it's easily ruined by bright lights or being tired.
Full Definition
The ability of the eye to see in low-light conditions, relying primarily on the rod cells of the retina rather than the cones used in daylight. Night vision develops gradually as the eyes adapt to darkness, is centered on the peripheral (off-center) field of view rather than directly ahead, and is degraded by exposure to bright light, low oxygen levels at altitude, fatigue, and certain substances such as tobacco and alcohol.