An elastic body wave or sound wave. It is the wave most studied in reflection and refraction seismology. Particles oscillate about a fixed point, but in the direction of propagation of wave energy. In an isotropic and homogeneous medium the P-wave velocity (Vp) is given by: Vp = [ψ/ρ]½, where ψ is the axial modulus and ρ is the density of the material. P-waves are the fastest of the seismic waves.