The Collaborative International Dictionary of English v.0.48:

Metamer \Met"a*mer\, n. [See Metamere.] (Chem.)
   Any one of several metameric forms of the same substance, or
   of different substances having the same composition; as,
   xylene has three metamers, viz., orthoxylene, metaxylene, and
   paraxylene; an isomer.
   [1913 Webster]