An Etymological Dictionary of Astronomy and Astrophysics
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Cosmic rays which typically have energies in the range 10¹⁵ to 10²⁰ electron volts. For the most part, they are protons and other atomic nuclei, and come from distant cosmos, perhaps even from outside our own Galaxy.

→ high; → energy; → cosmic; → ray.

A particle belonging to the most energetic population of → cosmic rays with an energy above ~ 10²⁰ → electron-volts. The UHECRs constitute a real challenge for theoretical models, because their acceleration requires extreme conditions hardly fulfilled by known astrophysical objects. See also → UHECR puzzle, → Greisen-Zatsepin-Kuzmin cutoff.

→ ultra- + → high-energy cosmic ray.