P-wave 
(pēwāv′)
(pēwāv′)n.
A seismic wave that travels very quickly through the earth, causing the rocks it passes through to change in volume and their particles to vibrate parallel to the direction of wave propagation. P-waves can travel through solids, liquids, and gases.
[P(rimary) wave, since it is the first type of wave to be detected by a seismograph after a seismic event.]