光通常指的是人类眼睛可以见的电磁波（可见光谱），视知觉就是对于光的知觉。光只是电磁波谱上的某一段频谱，一般是定义为波长介于400至700奈米（nm）之间的电磁波，也就是波长比紫外线长，比红外线短的电磁波。有些数据源定义的可见光的波长范围也有不同，较窄的有介于420至680nm，较宽的有介于380至800nm。

而有些非可见光也可以被称为光，如紫外光、红外光、x光。

光既是一种高频的电磁波，又是一种由称为光子的基本粒子组成的粒子流。因此光同时具有粒子性与波动性，或者说光具有“波粒二象性”。

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is visible to the human eye and is responsible for the sense of sight. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), or 4.00 × 10 to 7.00 × 10 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths). This wavelength means a frequency range of roughly 430–750 terahertz (THz).