An electronic oscillator is an electronic circuit that produces a repetitive electronic signal, often a sine wave or a square wave.

A crystal oscillator is an electronic circuit that uses the mechanical resonance of a vibrating crystal of piezoelectric material to create an electrical signal with a very precise frequency. This frequency is commonly used to keep track of time (as in quartz wristwatches), to provide a stable clock signal for digital integrated circuits, and to stabilize frequencies for radio transmitters.

Using an amplifier and feedback, it is an especially accurate form of an electronic oscillator. The crystal used therein is sometimes called a "timing crystal". On schematic diagrams a crystal is sometimes labeled with the abbreviation XTAL.