PAPI (Precision Approach Path Indicator)

A visual approach slope indicator consisting of a row of four light units located beside the runway, usually on the left side. The lights appear either red or white depending on the aircraft's position relative to the desired glidepath. Two red and two white lights indicate the pilot is on the correct 3-degree glidepath. More red means too low; more white means too high. The mnemonic is "red over white, you're all right."