Source : Webster's Revised Unabridged Dictionary (1913)

Isomeric \I`so*mer"ic\, a. [Iso- + Gr. me`ros part: cf. F.
   isom['e]rique.] (Chem.)
   Having the same percentage composition; -- said of two or
   more different substances which contain the same ingredients
   in the same proportions by weight, often used with with.
   Specif.:
   (a) Polymeric; i. e., having the same elements united in the
       same proportion by weight, but with different molecular
       weights; as, acetylene and benzine are isomeric
       (polymeric) with each other in this sense. See
       {Polymeric}.
   (b) Metameric; i. e., having the same elements united in the
       same proportions by weight, and with the same molecular
       weight, but which a different structure or arrangement of
       the ultimate parts; as, ethyl alcohol and methyl ether
       are isomeric (metameric) with each other in this sense.
       See {Metameric}.

Source : WordNet®

isomeric
     adj : of or relating to or exhibiting isomerism