Diode

A diode is an electronic component, which lets current flow in one direction, but blocks the current in the opposite direction. Thus, one speaks of the forward and reverse direction of a diode.

Diodes have a positive (+) and negative (-) terminal. The negative terminal (cathode) is clearly marked on both the component and in the circuit symbol...

Various diodes. (Enlarge)

Circuit symbol diode.

By the term "diode" today one generally thinks only of semiconductor diodes and silicon diodes. All other diodes are identified by an addition to the term (for example: Schotky diode, Zener diode, diode tube, etc).