Plain English Meaning

A set of lights next to the runway that tell the pilot, by changing color, whether the approach is too high, too low, or just right.

Full Definition

A ground-based system of lights placed beside the runway that gives the pilot a visual indication of whether the aircraft is on, above, or below the correct approach path to the landing threshold. A standard VASI uses two bars of lights that appear red or white depending on the aircraft's position relative to the glide path: white over white means too high, red over red means too low, and red over white means on the correct slope.