The GZK limit (for Greisen-Zatsepin-Kuzmin limit) is a theoretical limit on the kinetic 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 its three developers based upon the expected interaction of the particles with CMB photons if they travel through the CMB for a substantial distance, specifically, distances on the order of 50 kpc and above.
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.