 Eikonal function

The rapidly varying real phase of a short-wavelength field written $a(\mathbf{r},t)e^{iS(\mathbf{r},t)}$. Its derivatives define the local wavevector $\mathbf{k}=\nabla S$ and angular frequency $\omega =-{\partial }_{t}S$. The WKB approximation assumes that $a$ and background parameters vary little across one wavelength.