1) Theoretical chemistry 
Classical Isosteres are molecules or ions with the same number of atoms and/or the same number of valence electrons. The definition was later revised to include compounds with similarly reactive electron shells. For example, Hydrogen ion and Fluoride are classical isosteres because they both have relatively small nuclei and their outer valence she...
line connecting points of equal specific volume of a substance
One of two or more atoms or molecules having the same electron arrangement; e.g., N2 and CO. ... Origin: iso-+ G. Stereos, solid ... (05 Mar 2000) ... 
Type: Term Pronunciation: ī′sō-stēr Definitions: 1. One of two or more atoms or molecules having the same electron arrangement, N2 and CO.
a compound isosteric with one or more other compounds. · a line on a map or chart connecting points of equal atmospheric density.