Compression wave or longitudinal wave: shown travelling from left to right in a medium. The particles are first shoved forward by the pressure and then they return to their original positions (they oscillate in same the direction that the wave travels, hence longitudinal). This creates a region of higher pressure and density (compression) that travels along as a wave. This is immediately followed by a zone of lower pressure (or rarefaction, where the particles are further apart). This is how sound propagates in air or water and how p-waves travel during earthquakes.