What is "metamer"

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.

n. (context chemistry English) Either of a number of molecules that mutually exhibit metamerism.