定义：
单位长度的相位延迟，或者该值除以 2π。

对于波数有不同的定义。物理上的定义为： 

其中λ是介质中的波长（不是真空波长）。波数是波矢的大小，是平面波传播过程中单位长度的相位延迟。 
另一个定义： 

单位是cm−1，在光谱学中常用到。为了避免混淆，第一个量可以称为角波数（类似于角频率），但是这种叫法不常见。 
介质中光波的波数等于真空波数乘以折射率。 
波数与平面波在各向同性介质中传播时单位长度的相位改变有关。聚焦的光束单位长度的相位变化则需要在平面波的基础上做一些修正。例如，高斯光束的修正项为古依相移。对于在波导中传播的导波来说，传播常数γ（也称为β）的虚部是与单位长度的相位变化相关的。

Definition: the phase delay per unit length, or that quantity divided by 2π

Formula symbol: k, ν

Units: rad/m, cm⁻¹

Unfortunately, there are different definitions of the wavenumber of light in the literature. In physics, the definition

is common, where λ is the wavelength in the medium (not the vacuum wavelength). That (angular) wavenumber is the magnitude of the wave vector, and is the phase delay per unit length during propagation of a plane wave.

The other definition

(with units of cm⁻¹) is widely used in the field of spectroscopy and therefore called the spectroscopic wavenumber. The former quantity can be called angular wavenumber (in analogy with angular frequency) to avoid confusion, but that term is not very common.

For light in a medium, the wavenumber is the vacuum wavenumber times the refractive index. Spectroscopic wavenumbers are usually considered in vacuum.

The wavenumber is related to the phase change per unit length of a plane wave in a homogeneous medium. For focused beams, the phase change per unit length is modified with respect to that for a plane wave. For Gaussian beams, for example, this modification is the Gouy phase shift. For propagation of guided waves in waveguides, the imaginary part of the propagation constant γ (called β) is the relevant quantity.