passes a spark between two electrodes inside the glass envelope to generate light. Cannot be dimmed (in any meaningful way) electrically, and a mechanical dimmer must be used. A high voltage is needed to ionise the gas in the envelope in the first place ( striking'), whilst the voltage once the lamp has 'struck' must be carefully maintained. These functions are usually carried out by a ballast unit.
Discharge lamps are a type of light bulb that includes a discharge chamber with a discharge gas. The discharge gas responds to an arc or electron flow to create radiation. The walls of the discharge vessel may respond to the radiation of the gas to produce visible light.