noun
1. : a cycle of thermonuclear reactions that involves the synthesis of four hydrogen atoms into a helium atom with the release of nuclear energy and is held to be the source of most of the energy radiated by the sun and stars
2. : the cycle of carbon in living beings consisting of the uptake and fixing of carbon dioxide by photosynthesis, its consumption in carbohydrate, protein, and fat by animals and plants lacking photosynthesis (as in plant tissues eaten or organic matter absorbed), and its return to the inorganic state through respiratory processes and the decay of plant and animal bodies