In cosmology, the term relic (either as a noun or an adjective, with adjective primordial meaning basically the same thing) refers to some type of particle left over from an event, in the early universe, e.g., the phrases "relics from recombination", "relic neutrinos", and "primordial neutrinos". For example much hydrogen and helium is presumed to have been synthesized very soon after the Big Bang (Big Bang nucleosynthesis), in part because the reactions that would take place given the heat and density that would be present at the time would (according to established physics) produce abundances close to today's, i.e., the puzzle fits together.
The CMB is relic photons from recombination, left over from photons that were being constantly scattered (and emitted and absorbed) before that. That is an example of a decoupling (of photons from electrons and protons), and a freeze-out, Hubble expansion enlarging space sufficiently that the photons began free streaming (i.e., a significant number have been traveling ever since, with no end in sight). Relics include all or a significant fraction of these:
Those that are not relics have been transformed by subsequent processes and events, such as radioactive decay, stellar fusion, supernovae, and I'm guessing maybe active galactic nuclei.