A ballast is an electrical device placed in line with a load to limit current. They are widely used in lighting applications to limit and maintain current that passes through fluorescent or high intensity discharge (HID) lamps.
Because fluorescent lamps a negative differential resistance characteristics, ballasts are necessary to prevent the current passing through the tube from rising too high. Ballasts used for this purpose are inductive, meaning they store current in a magnetic field. Other types of ballasts can function more like resistors or capacitors, or some combination of all three.
Modern ballasts are electronic, using solid state technology to provide proper operating conditions needed to power the lamps. They are usually smaller and more efficient than their magnetic counterparts, and may allow dimming using pulse-width modulation technology and high frequency adjustments.