In electronics, a diode is a two-terminal electronic component with an asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. A vacuum tube diode is a vacuum tube with two electrodes, a plate (anode) and heated cathode.
The symbol used for a semiconductor diode in a circuit diagram specifies the type of diode. There are alternate symbols for some types of diodes, though the differences are minor.