The Collaborative International Dictionary of English v.0.48:

Isomer \I"so*mer\ ([imac]"s[-o]*m[~e]r), n. [See Isomeric.]
   (Chem.)
   A compound which is isomeric with another body or compound; a
   compound having the same chemical composition as another
   compound; a member of an isomeric series.
   [1913 Webster +PJC]

   Note: An isomer may be a structural isomer, in which some
         of the atoms are bonded to different atoms (as in
         CH3.CH2.NH2 and CH3.NH.CH3), or an optical
         isomer, in which the number and types of chemical
         bonds are identical, but the arrangement of the atoms
         in three-dimensional space is different; the latter are
         also called stereoisomers.
         [PJC]

WordNet (r) 3.0 (2006):

isomer
    n 1: a compound that exists in forms having different
         arrangements of atoms but the same molecular weight

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "isomer":
   acid, acidity, agent, alkali, alkalinity, alloisomer, anion,
   antacid, artificial isotope, atom, base, biochemical, cation,
   chemical, chemical element, chromoisomer, compound, copolymer,
   deuterium, dimer, element, heavy chemicals, heavy hydrogen,
   high polymer, homopolymer, hydracid, inorganic chemical, ion,
   isobar, isotone, isotope, macromolecule, metamer, molecule,
   monomer, neutralizer, nonacid, nuclear isomer, organic chemical,
   oxyacid, polymer, protium, pseudoisomer, radical,
   radioactive isotope, radioisotope, reagent, strontium 90, sulfacid,
   trimer