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.
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