[ uhn-stey-buhl ]
/ 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.


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Origin of unstable

Middle English word dating back to 1175–1225; see origin at un-1, stable2


2, 3 See unsettled.
3 vacillating.


un·sta·ble·ness, nounun·sta·bly, adverb
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2020

Example sentences from the Web for unstable

British Dictionary definitions for unstable

/ (ʌnˈsteɪbəl) /


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; radioactivean unstable nuclide
electronics (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation

Derived forms of unstable

unstableness, noununstably, 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

Scientific definitions for unstable

[ ŭn-stābəl ]

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.
Relating to a chemical compound that readily decomposes or changes into other compounds or into elements.
Relating to an atom or chemical element that is likely to share electrons; reactive.
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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.