unstable

adjective

un·​sta·​ble ˌən-ˈstā-bəl How to pronounce unstable (audio)
Synonyms of unstable
: not stable : not firm or fixed : not constant: such as
a
: not steady in action or movement : irregular
an unstable pulse
b
: wavering in purpose or intent : vacillating
c
: lacking steadiness : apt to move, sway, or fall
an unstable tower
d(1)
: liable to change or alteration
an unstable economy
unstable weather
(2)
: readily changing (as by decomposing) in chemical or physical composition or in biological activity
e
: characterized by lack of emotional control
unstableness noun
unstably
ˌən-ˈstā-b(ə-)lē How to pronounce unstable (audio)
adverb

Synonyms of unstable

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inconstant, fickle, capricious, mercurial, unstable mean lacking firmness or steadiness (as in purpose or devotion).

inconstant implies an incapacity for steadiness and an inherent tendency to change.

an inconstant friend

fickle suggests unreliability because of perverse changeability and incapacity for steadfastness.

performers discover how fickle fans can be

capricious suggests motivation by sudden whim or fancy and stresses unpredictability.

an utterly capricious critic

mercurial implies a rapid changeability in mood.

made anxious by her boss's mercurial temperament

unstable implies an incapacity for remaining in a fixed position or steady course and applies especially to a lack of emotional balance.

too unstable to hold a job

Examples of unstable in a Sentence

an unstable nuclear reactor core the minute we put the books down on the unstable desk, the whole stack went crashing to the floor
Recent Examples on the Web
Examples are automatically compiled from online sources to show current usage. Read More Opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback.
As Anna covers for Fyodor’s addiction and increasingly unstable professional behavior, her devotion starts to look poisonous for them both. — Alison Foreman, IndieWire, 5 Sep. 2026 Because a given centaur's orbit crosses the path of at least one of Jupiter, Saturn, Uranus and Neptune, a centaur's orbit is unstable over millions of years. — Keith Cooper, Space.com, 4 Sep. 2026 Sorghum is rich in protective plant compounds, such as polyphenols and other antioxidants, that help neutralize unstable molecules (free radicals) that can damage cells and promote inflammation over time. — Amy Brownstein, Verywell Health, 4 Sep. 2026 Online, many trial observers seized on those gaps, claiming Clancy was failed by a system that expected an unstable patient to carry her critical information from one provider to the next. — Nancy Dillon, Rolling Stone, 4 Sep. 2026 See All Example Sentences for unstable

Word History

First Known Use

13th century, in the meaning defined above

Time Traveler
The first known use of unstable was in the 13th century

Phrases Containing unstable

Rhymes for unstable

See All Rhymes for unstable

Browse Nearby Words

unstability
unstable
unstable angina
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Cite this Entry

“Unstable.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/unstable. Accessed 7 Sep. 2026.

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Kids Definition

unstable

adjective
un·​sta·​ble ˌən-ˈstā-bəl How to pronounce unstable (audio)
ˈən-
1
: not stable : not firm or fixed : fluctuating
unstable prices
an unstable tower
unstable weather
2
: readily changing in chemical composition or physical state or properties (as by breaking down into parts)
an unstable atomic nucleus

Medical Definition

unstable

adjective
un·​sta·​ble -ˈstā-bəl How to pronounce unstable (audio)
: not stable: as
a
: characterized by frequent or unpredictable changes
a patient in unstable condition
b
: readily changing (as by decomposing) in chemical composition or biological activity
unstable compounds
c
: characterized by lack of emotional control or stability
Last Updated: - Updated example sentences
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