• synonyms


[ahy-som-uh-riz-uh m]
  1. Chemistry. the relation of two or more compounds, radicals, or ions that are composed of the same kinds and numbers of atoms but differ from each other in structural arrangement (structural isomerism), as CH3OCH3 and CH3CH2OH, or in the arrangement of their atoms in space and therefore in one or more properties.Compare optical isomerism, stereoisomerism.
  2. Also called nuclear isomerism. Physics. the relation of two or more nuclides that have the same atomic number and mass number but different energy levels and half-lives.
  3. Chemistry, Physics. the phenomenon characterized by such a relation.
  4. the state or condition of being isomerous.
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Origin of isomerism

First recorded in 1830–40; isomer(ic) + -ism
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

Examples from the Web for isomerism

Historical Examples

  • One of the most remarkable phenomena of chemistry is that of isomerism.

    The Making of Species

    Douglas Dewar

  • Among organisms we sometimes observe a phenomenon which looks very like isomerism.

  • A rough illustration of the phenomenon of isomerism is furnished by written language.

  • A further distinction is necessary to a survey of the subdivisions of isomerism regarded in its widest sense.

  • Before entering into detail it may be useful to consider the nature of isomerism from a general standpoint.

British Dictionary definitions for isomerism


  1. the existence of two or more compounds having the same molecular formula but a different arrangement of atoms within the moleculeSee also stereoisomerism, optical isomerism
  2. the existence of two or more nuclides having the same atomic numbers and mass numbers but different energy states
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Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

isomerism in Medicine


  1. The phenomenon of the existence of isomers.
  2. The complex of chemical and physical phenomena characteristic of or attributable to isomers.
  3. The state or condition of being an isomer.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.