It is an electronic component that restrict flow of charge carriers.
It allows the current to flow in one direction but blocks it in another direction.
Diodes are commonly made from either Silicon or Germanium.
Thermionic diodes are thermionic valve devices (also known as vacuum tubes), which are arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appearance to incandescent light bulbs.
Most modern diodes are based on semiconductor p-n junctions. In a p-n diode, conventional current can flow from the p-type side (the anode) to the n-type side (the cathode), but cannot flow in the opposite direction. Another type of semiconductor diode, the Schottky diode, is formed from the contact between a metal and a semiconductor rather than by a p-n junction.
Diodes that can be made to conduct backwards. This effect, called Zener breakdown, occurs at a precisely defined voltage, allowing the diode to be used as a precision voltage reference. In practical voltage reference circuits Zener and switching diodes are connected in series and opposite directions to balance the temperature coefficient to near zero.
It can be used as solar cells and in photometry.
Used in making sign boards or for measuring temperature etc.