A simple wave or progressive disturbance in the isentropic flow of a compressible fluid, such that the pressure and density of a fluid particle decrease on crossing the wave in the direction of its motion. This may be illustrated, for example, by the withdrawal of a piston from a gas-filled cylinder. When the gas is initially at rest in the cylinder, an expansion wave may move into the undisturbed fluid at the speed of sound as the piston is withdrawn. The expansion wave is a finite-amplitude disturbance changing shape as it propagates, but it may or may not be accompanied by a shock wave front, with the associated discontinuities. See compression wave.