1. Ecol.the circulation of carbon atoms in the biosphere as a result of photosynthetic conversion of carbon dioxide into complex organic compounds by plants, which are consumed by other organisms: the carbon returns to the atmosphere in the form of carbon dioxide as a result of respiration, decay by fungi, bacteria, etc., and combustion of fossil fuels.
2. Astrophysics.a cycle of nuclear transformations in the interiors of the stars by means of which hydrogen is gradually converted into helium with the release of nuclear energy.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.