A primary wave, or P-wave, is a compressional seismic wave that travels through the body of the Earth when an earthquake occurs. It is the first wave to be measured by a seismograph and can be used in conjunction with the secondary wave, or S-wave, to tell seismologists important things about an earthquake.

P-waves travel extremely fast and have very low amplitude. They are referred to as "compressional" because particles in the Earth move parallel to the direction of the wave traveling through them.