unstable

adjective

un·​sta·​ble ˌən-ˈstā-bəl 

: 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 

adverb

Synonyms of unstable

• unsteady
• precarious
• rocky
• unbalanced

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

In the meantime, an unstable government could become more rather than less aggressive, not least to keep younger hard-liners from rebelling. —Arkansas Online, 10 Feb. 2026

This is clear on entry, as the exhibition floor itself is turned into a series of unstable ramps that rise and fall, echoing the references to the promises and disappointments of modern life that permeate the show. —K. Desbouis, Artforum, 6 Feb. 2026

Like all successful paranoid art, the core of the story is unstable. —James Folta, Literary Hub, 5 Feb. 2026

This causes a blast of high-energy radiation called a gamma-ray burst (GRB), a final screech of gravitational waves, and sends out a spray of neutron-rich matter, which allows a process to occur that generates very heavy but unstable elements. —Robert Lea, Space.com, 4 Feb. 2026

Word History

First Known Use

13th century, in the meaning defined above

Phrases Containing unstable

• unstable angina

Rhymes for unstable

• able
• chaebol
• fable
• label
• sable
• stabile
• stable
• table
• allocable
• educable
• integrable
• purchasable

Kids Definition

unstable

adjective

un·​sta·​ble ˌən-ˈstā-bəl 

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 

: 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