A phase-locked loop (PLL) is an electronic circuit that compares the phase of an input reference signal to a signal derived from its output oscillator. It adjusts the oscillator's frequency to keep the input and output phases matched. This allows the PLL to track the input frequency or generate a frequency that is a multiple of the input. PLLs are widely used to generate stable frequencies, recover signals, and distribute clock timing in electronic applications such as radio, telecommunications, and computers. They can operate at frequencies ranging from fractions of a hertz to many gigahertz.