A simple wave or progressive disturbance in the one-dimensional isentropic flow of a compressible fluid, such that the pressure and density of a fluid particle increase on crossing the wave in the direction of its motion. A compression wave is illustrated, for example, by the compression of gas in a cylinder by means of a piston. When the gas is initially at rest in the cylinder, a compression wave may move into the undisturbed fluid at the speed of sound as the piston is advanced.