The oscillatory motion of the particles of a medium caused by the passage of a wave, produced by forces external to the medium, but propagated through the medium by internal forces. Wave motion per se involves no net translation of the medium. Various types of oscillation are found in natural wave motions. Among the simplest are the linear oscillation parallel to the direction of propagation of a longitudinal wave, the linear oscillation perpendicular to the direction of propagation of a transverse wave, and the orbital motion produced by the passage of a progressive gravity wave.