定义：
描述波传播方向和单位长度的相位延迟的矢量。

平面波的波矢（k矢量）指向波传播的方向，通常垂直于波前。它的大小是波数，定义为： 

其中λ是介质中的波长（不是真空波长）。 
在非各向同性介质中，能流的方向与波矢方向可能不同，这种现象称为随机游走。 
在吸收或增益介质中，波矢可能具有复数成分。在衰逝场情形（参阅全反射），波矢甚至可以为纯虚数。

Definition: a vector indicating the direction of wave propagation and the phase delay per unit length

The wave vector (or k vector) of a plane wave is a vector which at least in case of isotropic optical media points in the direction in which the wave propagates. It is always perpendicular to the wavefronts.

The magnitude of the wave vector (with units of m⁻¹) is the wavenumber as defined by

where λ is the wavelength in the medium (not the vacuum wavelength).

In non-isotropic media, the direction of energy flow, which is the direction of the Poynting vector, can somewhat deviate from that of the wave vector, which is always perpendicular to the wavefronts. This phenomenon is called spatial walk-off.

In media with absorption or gain, the wave vector can have complex components. In case of an evanescent wave, it can even have a purely imaginary component.