定义：
光（或者微波）的谐振腔。

光学谐振腔通常称为腔。这一概念来源于微波技术中，其中谐振腔确实类似于一个闭合的强，而光学谐振腔通常则是开放的。
几何结构的不同也与光学谐振腔与光波长相比通常很大有关，而微波腔不会比波长大很多。 
只有光学微腔尺度在微米甚至亚微米量级，可以在任意方向上约束光场。微腔可由半导体或者玻璃结构实现，例如微型环芯，或者光子晶体中的缺陷结构。 
尽管腔这一概念不是完全合适，许多表达方式都是以其为基础。例如激光腔（激光器谐振腔），腔倾斜和腔设计（谐振腔设计）。 
两种基本的腔为： 

• 驻波（线性）腔，其中两束光被两端镜子来回反射 
• 环形腔，光可以在其中沿不同方向做往返运动

Definition: resonators for light (or for microwaves)

Alternative term: resonators

Optical resonators are often called cavities. This term has been taken over from microwave technology, where resonators really look like closed cavities, whereas optical resonators normally have an “open” kind of setup, with reflecting surface only at few locations. That difference in geometry is related to the fact that optical resonators are usually very large compared with the optical wavelength, whereas microwave cavities are often not much longer than a wavelength, so that diffraction effects are much stronger.

Only so-called optical microcavities have dimensions in the micrometer or even sub-micrometer regime, and can tightly enclose a light field in all directions. Microcavities can be realized e.g. with tiny semiconductor or glass structures, such as microtoroids, or as defect structures in photonic crystals.

There are also spherical resonators, supporting the so-called whispering gallery modes; for such resonators, the term cavities is also quite appropriate.

Even though the term “cavity” is often not completely appropriate, as explained above, many related terms are based on it. Examples are laser cavities (laser resonators), cavity modes (resonator modes), cavity dumping and cavity design (resonator design).

The two basic types of optical cavities are:

• standing-wave (or linear) cavities, where light bounces back and forth between two end mirrors
• ring cavities (ring resonators), where the light can do round trips in two different directions

The article on optical resonators contains more details.