An optical resonator is a system or set up, which is used to obtain amplification of stimulated photons by oscillating them back and forth between system of two mirrors. Thus, it consists of two plane or concave mirrors. One of the mirrors is partially reflecting (having reflectivity less than 100%) and other is totally reflecting (having reflectivity 100%).
Laser output is received from partially reflecting mirror. The space between the two mirrors is called cavity.
Note: The post is referred from my book “Optics & Lasers”.