Simple wave

From Glossary of Meteorology

simple wave

The solution of a set of quasi-linear differential equations describing a fluid flow that possesses a family of straight-line characteristics.

In such a fluid motion the dependent variables, or simple combinations thereof, are constant along the characteristics, and may be used as the basis of the integration of the equations by the method of characteristics. The simple waves of most interest to meteorology are the expansion wave and the compression wave.
See simple harmonic wave.