unstable

American  

[uhn-stey-buhl] / ʌnˈsteɪ bəl /

adjective

1. not stable; not firm or firmly fixed; unsteady.

2. liable to fall or sway.

   Synonyms:

   precarious

3. unsteadfast; inconstant; wavering.

   unstable convictions.

   Synonyms:

   vacillating

4. marked by emotional instability.

   an unstable person.

5. irregular in movement.

   an unstable heartbeat.

6. Chemistry. noting compounds that readily decompose or change into other compounds.

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unstable British  

/ ʌnˈsteɪbəl /

adjective

1. lacking stability, fixity, or firmness

2. disposed to temperamental, emotional, or psychological variability

3. (of a chemical compound) readily decomposing

4. physics

   1. (of an elementary particle) having a very short lifetime

   2. spontaneously decomposing by nuclear decay; radioactive

      an unstable nuclide

5. electronics (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation

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unstable Scientific  

/ ŭn-stā′bəl /

1. 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.

2. See more at decay

3. Relating to a chemical compound that readily decomposes or changes into other compounds or into elements.

4. Relating to an atom or chemical element that is likely to share electrons; reactive.

5. Characterized by uncertain or inadequate response to treatment and the potential for unfavorable outcome, as the status of a medical condition or disease.

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Related Words

See unsettled.

Other Word Forms

• unstableness
  noun
• unstably
  adverb

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.

Example Sentences

A Hypershell spokesman said that a snug, proper fit and a lower level of assistance can help on unstable terrain.

From The Wall Street Journal • May 21, 2026

Also, I have flashbacks of panic due to his previously unstable financial behavior.

From MarketWatch • May 16, 2026

She did some moves that were pretty irrational, and she would have times where she would act on impulse, and it became more and more unstable and unsafe.

From Los Angeles Times • May 13, 2026

Mohammadi's Paris-based husband said "she is not in a favourable general condition" and that "her status remains unstable", in a statement over the weekend.

From BBC • May 10, 2026

In fact, it could well be the case that life, once it becomes complex, is inherently unstable and will self-destruct within a few thousand years.

From "Innumeracy: Mathematical Illiteracy and Its Consequences" by John Allen Paulos

Related Words

• ambiguous
• changeable
• erratic
• fickle
• insecure
• irrational
• precarious
• risky
• rocky
• sensitive
• shaky
• slippery
• ticklish
• tricky
• uncertain
• unpredictable
• unsettled
• unsteady
• volatile
• weak