The term p-process (p for proton) is used in two ways in the scientific literature concerning the astrophysical origin of the elements (nucleosynthesis).
Originally it referred to a proton capture process which was proposed to be the source of certain, naturally occurring, neutron-deficient isotopes of the elements from selenium to mercury.
At the same time the ratio of protons to neutrons is changed, resulting in a more neutron-deficient isotope of the next element.
The higher the Coulomb barrier, the more kinetic energy a proton requires to get close to a nucleus and be captured by it.
The authors assumed that this process was solely responsible for the p-nuclei and proposed that it occurs in the hydrogen-shell (see also stellar evolution) of a star exploding as a type II supernova.