The Collaborative International Dictionary of English v.0.48:

isomeric \i`so*mer"ic\ ([imac]`s[-o]*m[e^]r"[i^]k), a. [Iso- +
   Gr. me`ros part: cf. F. isom['e]rique.] (Chem.)
   Having the same chemical composition and molecular weight;
   having the same number of atoms of each kind in the molecule;
   -- said of chemical compounds with known chemical
   composition. This property used to be called metameric to
   distinguish it from other forms of isomerism.
   [PJC]

   Note: Formerly the term isomeric applied also to comopounds
         having the same percentage chemical composition, even
         if the number of atoms in a molecule differed. In the
         1913 dictionary isomeric was defined as:
         "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 with 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."
       [1913 Webster]

WordNet (r) 3.0 (2006):

isomeric
    adj 1: of or relating to or exhibiting isomerism