Source : Webster's Revised Unabridged Dictionary (1913)

Isomer \I"so*mer\, n. [See {Isomeric}.] (Chem.)
   A body or compound which is isomeric with another body or
   compound; a member of an isomeric series.

Source : WordNet®

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