1. Chemistry Any of two or more substances that have the same molecular formula but differ in the way their atoms are connected to each other, in the spatial orientation of their atoms, or, in the case of large molecules such as DNA, in their molecular topology.
2. Physics Any of two or more nuclei with the same mass number and atomic number that have different radioactive properties and can exist in any of several energy states for a measurable period of time.
[Greek īsomerēs, having equal share : īso-, iso- + meros, part, share; see (s)mer-2 in the Appendix of Indo-European roots.]
i′so·meric (-mĕrĭk) adj.
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2020 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Indo-European & Semitic Roots Appendices
Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:
The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.