A ballast is a device used in electrical circuits to regulate the current flowing to a light source, particularly in fluorescent and high-intensity discharge (HID) lamps. It provides the necessary voltage to start the lamp and then limits the current to prevent overheating and damage. In addition to lighting, ballasts are also found in other applications, such as in aquarium systems to control the lighting for aquatic life. They come in various types, including magnetic and electronic, each serving the same fundamental purpose but with different efficiencies and features.