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.