(High Intensity Discharge) A new type of headlight first seen on luxury automobiles. Instantly recognizable by their bluish-white tint, and much more bright than regular or halogen headlights. They do not have bulbs or tungsten filaments. Light is produced by a high-voltage arc between two electrodes in a gas capsule containing xenon. This is similar but not quite like sodium vapor street lighting.
Beware of knock-offs. Halogen lights with a blue tint are not the same, and are probably illegal, check your highway code.