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P-wave (compressional wave, dilatational wave, irrotational wave, pressure wave, primary wave, push—pull wave) 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.