1. A process in which an inner shell electron is captured by the nucleus of its own atom.

The mass number is unchanged, but the atomic number is decreased by one and this process is accompanied by the emission of a neutrino.

2. A radioactive decay process in which an atomic nucleus with an excess of protons draws an electron into itself, creating a neutron out of a proton and thus decreasing the atomic number by one.

Often the resulting nucleus is unstable and achieves stability by giving off a gamma ray.