An optocoupler (or optoisolator) is a semiconductor device that transfers electrical signals using light, providing isolation between input and output. It consists of an LED and a photodetector (phototransistor, photodiode, or phototriac) in a sealed package. Optocouplers are widely used in signal isolation, noise reduction, and interfacing high-voltage circuits with low-voltage control systems in power supplies, microcontrollers, and industrial automation.