unstable
Americanadjective
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not stable; not firm or firmly fixed; unsteady.
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liable to fall or sway.
- Synonyms:
- precarious
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unsteadfast; inconstant; wavering.
unstable convictions.
- Synonyms:
- vacillating
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marked by emotional instability.
an unstable person.
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irregular in movement.
an unstable heartbeat.
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Chemistry. noting compounds that readily decompose or change into other compounds.
adjective
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lacking stability, fixity, or firmness
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disposed to temperamental, emotional, or psychological variability
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(of a chemical compound) readily decomposing
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physics
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(of an elementary particle) having a very short lifetime
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spontaneously decomposing by nuclear decay; radioactive
an unstable nuclide
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electronics (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation
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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.
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See more at decay
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Relating to a chemical compound that readily decomposes or changes into other compounds or into elements.
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Relating to an atom or chemical element that is likely to share electrons; reactive.
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Characterized by uncertain or inadequate response to treatment and the potential for unfavorable outcome, as the status of a medical condition or disease.
Synonym Usage
See unsettled.
Other Word Forms
Etymology
Origin of unstable
First recorded in 1175–1225; Middle English; un- 1 + stable 2
Explanation
To be unstable is to lack stability, meaning things could change without warning, like an unstable bookshelf that is likely to fall down. If you are unstable emotionally, you might be ecstatically happy one minute and horribly depressed and angry the next. The adjective unstable means the opposite of stable. Stable is derived from the Latin base word stare meaning “to stand.” If a building, family, person, bridge, or chemical solution is stable it will “stand still,” even if something unexpected happens. On the other hand, if a person or thing is unstable and stress is applied, he, she, or it is likely to shift position, possibly in a dangerous way.
Vocabulary lists containing unstable
100 SAT Words Beginning with "U"
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Engineering - Introductory
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Civil Engineering
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Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
When a lower-mass star reaches the point where nuclear reactions in its core can no longer support it, the star becomes unstable and begins to pulsate.
From Science Daily ● Aug. 12, 2026
“Hot and unstable conditions will result in an enhanced risk of large vertical plume growth, increasing the risk of extreme fire behavior for interior areas.”
From Los Angeles Times ● Aug. 8, 2026
As a result, international relations are unstable and dangerous.
From Salon ● Aug. 7, 2026
He added: "Individuals, unstable moments and unfortunate episodes come and go. The institution, its mission and its responsibility towards world football continue."
From Barron's ● Aug. 5, 2026
All you need is the evidence of the ECG, blood pressure, fluid in the lungs, and unstable angina.
From "Blink" by Malcolm Gladwell
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
