Isomerism \I*som"er*ism\, n. (Chem.) The state, quality, or relation, of two or more isomeric substances.

Physical isomerism (Chem.), the condition or relation of certain (metameric) 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 l[ae]vo-tartaric acids. In such compounds there is usually at least one unsymmetrical carbon atom. See Unsymmetrical.

Source: Webster's Revised Unabridged Dictionary (1913)