Source : Webster's Revised Unabridged Dictionary (1913)

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 : WordNet®

isomerism
     n : the state of being an isomer; the complex of chemical and
         physical phenomena characteristic of isomers