P waves are longitudinal. They are also known as primary waves because they typically arrive at seismic stations before S waves, which are transverse waves. P waves are characterized by particle motion in the same direction as the wave propagation, which allows them to travel through solids, liquids, and gases. In contrast, S waves, or secondary waves, have particle motion perpendicular to the direction of wave propagation and can only travel through solids. read more >>

For seismic waves through the bulk material the longitudinal or compressional waves are called P waves (for "primary" waves) whereas the transverse waves are callled S waves ("secondary" waves). Since any material, solid or liquid (fluid) is subject to compression, the P waves can travel through any kind of material.read more >>