device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others

A resonator is something that resonates. This may make a particular frequency of a sound or other vibration become bigger.

Electromagnetics change

In electromagnetics a cavity resonator is a resonator composed of a space that is usually surrounded by a dielectric or metal which allows standing wave patterns at particular frequencies, eigenmodes, correlated to their size, shape, and material.

Music change

Most musical instruments have resonators. They are the parts which makes the sound louder. A vibraphone, for example, has long tubes underneath the keys. It also has a special effect: a mechanism which, when it is turned on, opens and shuts the resonators very quickly so that a trembling sound is heard.

The body of a violin is a resonator: without the body the sound of the string vibrating would hardly be heard, just as the sound of an elastic band stretched between two fingers can hardly be heard.