Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. Chemistry Any of two or more substances that are composed of the same elements in the same proportions but differ in properties because of differences in the arrangement of atoms.
  • n. 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. Any of two or more compounds with the same molecular formula but with different structure.
  • n. Any of two or more atomic nuclei with the same mass number and atomic number but with different radioactive properties.

from the GNU version of the Collaborative International Dictionary of English

  • n. A compound which is isomeric with another body or compound; a compound having the same chemical composition as another compound; a member of an isomeric series.

from The Century Dictionary and Cyclopedia

  • n. In chem., a compound that exhibits the properties of isomerism with reference to some other compound. Also isomeride.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. a compound that exists in forms having different arrangements of atoms but the same molecular weight

Etymologies

Greek īsomerēs, having equal share : īso-, iso- + meros, part, share; see (s)mer-2 in Indo-European roots.
(American Heritage® Dictionary of the English Language, Fourth Edition)
iso- + -mer (Wiktionary)

Examples

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