Definitions for "P-wave"
A primary, or compression, body seismic wave that travels through the depth of the earth.
Primary seismic waves. The fastest set of earthquake vibrations. They move through the Earth in compression and expansion motions (much like sound waves move through air). Called primary because they are the first recorded at a seismograph. Primary waves are able to travel through both solids and liquids.
A seismic wave that involved particle motion in the direction of propagation; the p stands for primary because this wave arrives first.