unstable

1 of 2

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

unstableness

2 of 2

noun

un·​stableness

¦ən+

: the quality or state of being unstable

Synonyms of unstable

Adjective

• 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

Adjective

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

Adjective

But in times of instability, that selectorate may be far more unstable than assumed and forming relations with the wider population may manage that risk. — Radu Magdin, Forbes.com, 10 July 2026

The phenomenon occurs when there is unstable air created by high and low-pressure systems coming together, and moisture forms high above the ground while the air near the ground remains dry. — Rick Hurd, Mercury News, 9 July 2026

Fourteen years later, the district has gone through layoffs, school closures and unstable leadership. — Howard Blume, Los Angeles Times, 9 July 2026

The work was difficult, as the rescuers contended with unstable buildings and structures that could collapse at any moment. — Ella Moore july 9, Miami Herald, 9 July 2026

Word History

Etymology

Noun

Middle English unstablenesse, from unstable + -nesse -ness

First Known Use

Adjective

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
• irrigable
• 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