P wave

noun2

1. •
   A longitudinal compressional wave produced by an earthquake.

   • P-waves are the first to arrive; that is, they travel fastest. These are ‘push-pull’ waves analogous to sound waves, in which each particle moves back and forth as the waves propagate. — 2004
2. •
   A wave on an electrocardiogram that represents the electrical changes generated by the depolarization of the atria.