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 (V_p) is given by: V_p = [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.