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

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 Airlines typically advise travelers to do this, and only instruct them to sit down and buckle up during unstable weather. Angus Whitley / Bloomberg, TIME, 24 May 2024 The Thwaites Glacier, part of the vast West Antarctic Ice Sheet, is one of the world's fastest-changing and most unstable glaciers. Doyle Rice, USA TODAY, 20 May 2024 In character, Elordi is attempting to look as unstable (and, frankly, homosexual) as possible to avoid enlisting into military service. Matt Donnelly, Variety, 18 May 2024 Pelosi was viciously attacked in his home by a mentally unstable man with a hammer. Jay Nordlinger, National Review, 16 May 2024 See all Example Sentences for unstable 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'unstable.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

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

Dictionary Entries Near unstable

Cite this Entry

“Unstable.” Merriam-Webster.com Dictionary, Merriam-Webster, https://www.merriam-webster.com/dictionary/unstable. Accessed 18 Jun. 2024.

Kids Definition


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

Medical Definition


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

More from Merriam-Webster on unstable

Last Updated: - Updated example sentences
Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!