$\underset{\_}{\underset{\_}{k}}=\left(\text{j}\omega /c_0;\overrightarrow{k}\right)$

where $\omega$ is the angular frequency, $\text{j}$ is the imaginary unit, and $c_0$ is the speed of light in vacuum

Note 1 to entry: The coherent SI unit of four-wave vector is metre to the power minus one, ${\text{m}}^{\mathrm{-1}}$.