A bulky electronic device used to provide a smooth rise in voltage to fluorescent lamps when they are started, and insure consistent power while the lamp is in operation. Florescent and high-intensity discharge (HID) lamps both benefit from the use of ballasts.

It is widely believed that fluorescent and HID lamps must use ballasts, but they can both operate without them. Many low-cost, low-power consumer-grade HID and fluorescent fixtures are sold without ballasts. However, the stress created from instant voltage surges when the lamps are powered up, combined with the lack of filtering in an unballasted fixture, can significantly reduce the life expectancy and increase the energy consumption of a fluorescent tube, and cause premature failure in expensive HID lamps.

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