Diode
From DT Online
Diodes fall broadly into two types - rectifier and signal. Rectifier diodes are used in power supplies to convert a.c. to d.c.
They may have to handle high currents and voltages but normally work at low frequencies (typically 50 Hz).
Signal diodes normally handle much lower power but may be required to operate at much higher frequencies (many MegaHertz).
Diodes allow current to pass through them in one direction only.
In the forward direction, the resistance is low (typically 5 ohms - measured on an analogue ohmmeter).
In the reverse direction, the resistance is very high. (Greater than 20 megohms).
Uses include: rectifiers in power supplies, signal demodulators and back EMF limiters in inductors.