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Electron capture is a nuclear process consisting of a proton within an atom's nucleus combining with an electron in an inner electron shell, reducing the nucleus's positive charge by one, changing one proton to a neutron. It is the same thing as (or is a type of) neutronization. It happens spontaneously in certain radioactive elements. It can be (or perhaps always is) assisted by quantum tunneling. Electron capture always results in an emitted neutrino, a mechanism by which supernovae produce neutrinos.
The term electron capture is generally used for the capture of orbiting electrons, but combination with a free electron is also possible if the free electron has sufficient kinetic energy to make up the difference between the proton's and the resulting neutron's mass. The latter can happen at extreme temperatures, e.g., the center of large stars, or during supernovae.