HID

HID is short for High Intensity Discharge - a type of lamp.

HID lamps are very high energy efficient, high intensity type of lighting that is superior to halogen lamps. These are most often used in reef aquariums where coral requires intense light.

The light is produced by producing an electrical discharge between two electrodes inside a bulb filled with gas and metals. Special safety light fixtures are required for HID lamps as they operate under high pressure, run at a high temperature and emit high levels of potentially harmful ultra-violet light.

Links

 * HID by Wikipedia.