A two-terminal device having a low resistance to electric current in one direction and a high resistance in the reverse direction. Diodes are thus useful as rectifiers, converting alternate current (AC) into direct current (DC). Although electron-tube diodes were once common, almost all diodes today are semiconductor devices.

In general, current flowing thorough a diode is not proportional to the voltage between its terminals. When the voltage applied in the reverse direction exceeds a certain value, a semiconductor diode breaks down and conducts heavily in the direction of normally high resistance. This effect can be exploited to regulate voltage.

Some diodes are sensitive to light. A light emitting diode produces light as current passes through it; some LEDs can act as lasers. A thermistor is a special semiconductor diode whose conductivity increases with the diode temperature.