A longitudinal wave is a wave in which particles are displaced parallel to the direction the wave travels.
Longitudinal waves are characterized by regions called compressions, where the particles are closest together and rarefaction is where the particles are furthest apart.
A sound wave traveling through air is a classic example of a longitudinal wave. Sound wave are also mechanical waves that require material medium for propagation.
Sound wave sin are are longitudinal waves because air molecules move in a direction that is parallel to the direction that the wave moves
