Plain English Meaning

A set of lights next to the runway that tells you, just by their colour, whether your descent path is too high, too low, or just right. Red over white means you're on the correct slope. The phrase pilots use to remember it is 'red over white, you're alright.'

Full Definition

A ground-based system of lights installed beside the runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct approach slope to the runway. A standard VASI uses two bars of lights (near and far). When the pilot sees red over white, the aircraft is on the proper glide path. White over white indicates the aircraft is too high; red over red indicates too low. VASI guidance is normally usable from about 4 nautical miles out and provides within plus or minus 10 degrees of the runway .