- a semiconductor device with 2 terminals that controls the direction of [[Electrons]] in an [[Electrical Circuits|Electrical Circuit]] - they have polarity, with positive end ([[Anode]]) & negative end ([[Cathode]]) - converts [[Alternating Current|AC]] to [[Direct Current|DC]] - protects [[Electrical Circuits|Circuits]] from damage caused by reversing [[Current|Currents]] or [[Voltage]] spikes