Isomerism n. (Chem.) The state, quality, or relation, of two or more isomeric substances.
Physical isomerism (Chem.), a former term for optical isomerism.
optical isomerism (Chem.), the condition or relation of certain (isomeric) substances, which, while chemically identical (in that they have the same composition, the same molecular weights, and the same ultimate constitution), are yet physically different, as in their action on polarized light, as dextro- and laevo-tartaric acids, or the sugars galactose and glucose. In such compounds there is usually at least one unsymmetrical carbon atom and typically more than one. See Unsymmetrical.