The GZK limit (for Greisen-Zatsepin-Kuzmin limit) is a theoretical limit on the energy of cosmic rays (high velocity protons and other atomic nuclei), i.e., 5×1019 eV or about 8 joules. The limit was calculated by the three developers based upon the expected interaction of the particles with the cosmic microwave background over long distances, specifically, distances over about 50 kpc.
Cosmic rays above this limit have been observed and are called extreme-energy cosmic rays or EECRs. It is assumed they are from within 50 kpc. There is no consensus regarding the mechanism that creates them.