[¦kär·bən ¦nī·trə·jən ¦äk·sə·jən ‚sī·kəlz] (nuclear physics) A group of nuclear reactions involving the interaction of protons with carbon, nitrogen, and oxygen nuclei; completion of any of the cycles results in the consumption of four protons and the production of a helium-4 nucleus, two positrons, two neutrinos, and energy. Abbreviated CNO cycles.