A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal, typically made of quartz, to generate a precise frequency signal. This frequency is stable and can be used to keep time in devices like clocks and watches, as well as in communication systems. The crystal works by vibrating at a specific frequency when an electric current is applied. This property makes crystal oscillators essential in various applications, including radio transmitters, computers, and GPS devices, where accurate timing is crucial for performance and reliability.