A cavity resonator uses resonance to amplify a wave. The cavity has interior surfaces which reflect one type of wave. When a wave that is resonant with the cavity enters, it bounces back and forth within the cavity, with low loss (See standing wave). As more wave energy enters the cavity, it combines with and reinforces the standing wave, increasing its intensity.

Some common examples of cavity resonators include the klystron tube in a microwave oven (see also magnetron), the tube of a flute, and the body of a violin (this latter also being an example of a Helmholtz resonator).

In a laser, light is amplified in a cavity resonator which is usually composed of two or more mirrors. Thus an optical cavity, also known as a resonator, is a cavity with walls which reflect electromagnetic waves (light). This will allow standing wave modes to exist with little loss outside the cavity.