motion in a medium in which, when particles are displaced, a force proportional to the displacement acts on the particles to restore them to their original position. If a material has the property of elasticity and the particles in a certain region are set in vibratory motion, an elastic wave will be propagated. For example, a gas is an elastic medium (if it is compressed and the pressure is then released, it will regain its former volume), and sound is transmitted through a gas as an elastic wave.