A term used to characterize the degree of reduction (or @O04362@) in atoms, molecules and ions. An atom in a molecule or an ion which has a low @O04365@. An element or atom in a compound can be reduced by the reaction of an element or compound with hydrogen, while it can be oxidized by reaction with oxygen. A reduced species can be formed also through the gain of electrons (either at the negative electrode in a cell or through transfer from another atom, ion or group of atoms in a chemical reaction). For example, the carbon atom in CH4 and other @H02889@ is in a reduced state, while the carbon in CO2 is in an oxidized state. Similarly the sulfur atom in H2S is in a reduced state while that in sulfuric acid (H2SO4) is in an oxidized state.