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[uhn-stey-buh l] /ʌnˈsteɪ bəl/
not stable; not firm or firmly fixed; unsteady.
liable to fall or sway.
unsteadfast; inconstant; wavering:
unstable convictions.
marked by emotional instability:
an unstable person.
irregular in movement:
an unstable heartbeat.
Chemistry. noting compounds that readily decompose or change into other compounds.
Origin of unstable
Middle English word dating back to 1175-1225; See origin at un-1, stable2
Related forms
unstableness, noun
unstably, adverb
2. precarious. 2, 3. See unsettled. 3. vacillating. Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2017.
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Examples from the Web for unstable
Contemporary Examples
Historical Examples
  • As a jailer he was in close touch with facts and knew by experience how unstable in these days was any man's power.

    The Light That Lures Percy Brebner
  • Man in sooth is a marvellous, vain, fickle, and unstable subject.

    Familiar Quotations John Bartlett
  • Hence Napoleon was driven more and more to trust to the advice of the rash, unstable King of Naples.

    Napoleon's Marshals R. P. Dunn-Pattison
  • The world had become a thing of hideous flux, unstable as when first it was made.

  • Matter in an unstable condition tends either to explode or to grow or to disintegrate.

    The Breath of Life John Burroughs
British Dictionary definitions for unstable


lacking stability, fixity, or firmness
disposed to temperamental, emotional, or psychological variability
(of a chemical compound) readily decomposing
  1. (of an elementary particle) having a very short lifetime
  2. spontaneously decomposing by nuclear decay; radioactive: an unstable nuclide
(electronics) (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation
Derived Forms
unstableness, noun
unstably, adverb
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
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Word Origin and History for unstable

early 13c., "apt to move," from un- (1) "not" + stable (adj.). Cf. Middle High German unstabel. Meaning "liable to fall" is recorded from c.1300; sense of "fickle" is attested from late 13c. An Old English word for this was feallendlic, which might have become *fally.

Online Etymology Dictionary, © 2010 Douglas Harper
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unstable in Science
  1. Liable to undergo spontaneous decay into some other form. For example, the nucleus of uranium 238 atom is unstable and changes by radioactive decay into the nucleus of thorium 234, a lighter element. Many subatomic particles, such as muons and neutrons, are unstable and decay quickly into other particles. See more at decay.

  2. Relating to a chemical compound that readily decomposes or changes into other compounds or into elements.

  3. Relating to an atom or chemical element that is likely to share electrons; reactive.

  4. Characterized by uncertain or inadequate response to treatment and the potential for unfavorable outcome, as the status of a medical condition or disease.

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