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a combining form meaning “equal,” used in the formation of compound words: isochromatic; in chemistry, used in the names of substances which are isomeric with the substance denoted by the base word:
isocyanic acid.
Also, especially before a vowel, is-.
Origin of iso-
< Greek, combining form of ísos equal Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2018.
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British Dictionary definitions for iso-


combining form
equal or identical: isomagnetic
indicating that a chemical compound is an isomer of a specified compound: isobutane, isocyanic acid
Word Origin
from Greek isos equal
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin and History for iso-

word-forming element meaning "equal, similar, identical; isometric," from comb. form of Greek isos "equal to, the same as" (e.g. isometor "like one's mother"). Used properly only with words of Greek origin; the Latin equivalent is equi- (see equi-).

Online Etymology Dictionary, © 2010 Douglas Harper
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iso- in Medicine

iso- or is-

  1. Equal; uniform: isobar.

  2. Isomeric: isopropyl.

  3. Characterized by sameness with respect to species: isoantigen.

  4. Characterized by sameness with respect to genotype: isograft.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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iso- in Science
  1. A prefix that means "equal," as in isometric, "having equal measurements."

  2. A prefix used to indicate an isomer of an organic compound, especially a branched isomer of a compound that normally consists of a straight chain. The isomer is characterized by a Y-shaped branch at the end of the chain that consists of two "prongs". Each prong consists of one carbon atom. Thus isopentane contains five carbon atoms like normal pentane, but arranged as a chain of three carbons plus a Y-shaped branch of two carbons at the end.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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