A longitudinal wave is a type of wave in which the particles of the medium through which the wave travels vibrate in the same direction as the wave's propagation. In other words, the displacement of the medium's particles is parallel to the direction of the wave's energy transfer. This is in contrast to transverse waves, where the displacement of the medium's particles is perpendicular to the direction of the wave's travel. A common example of a longitudinal wave is a sound wave.
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