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 to synchronize signals. The crystal works by vibrating at a specific frequency when an electric current is applied. This property, known as piezoelectricity, allows the oscillator to produce a consistent output, making it essential in various applications, including computers, radios, and GPS devices.