- a device, as a two-element electron tube or a semiconductor, through which current can pass freely in only one direction.
- a semiconductor device containing one p-n junction, used in circuits for converting alternating current to direct current
- the earliest and simplest type of electronic valve having two electrodes, an anode and a cathode, between which a current can flow only in one direction. It was formerly widely used as a rectifier and detector but has now been replaced in most electrical circuits by the more efficient and reliable semiconductor diode
