A diode is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. This property makes diodes essential components in various electronic circuits, including rectifiers, which convert alternating current (AC) to direct current (DC). Diodes are used in many applications, from simple tasks like protecting circuits from voltage spikes to more complex roles in signal modulation and demodulation. They come in various types, such as Zener diodes for voltage regulation and LEDs (Light Emitting Diodes) for lighting and displays.