reduction, chemical reaction that is in effect the complement of oxidation and one in which an element or compound gains electrons. The essential feature of an oxidation is that electrons are transferred from the component being oxidized to the one being reduced. The reaction cannot be isolated; to have a reduction, something else must be oxidized. Common reduction reactions are those of iron ore to iron the metal, or the reduction of acetaldehyde to ethanol as happens in the final stage of alcoholic fermentation. Wine in a stoppered bottle or other airtight container is said to be in a reductive state.