isostere

from The Century Dictionary.

• noun A line connecting points of equal specific volume in the ocean or atmosphere: the inverse of isopyc. See isostath.
• noun In chem., a term applied to compounds possessing equal molecular volumes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry Either of a group of molecules or ions that have the same number of atoms and the same number of valence electrons