unstable

American  

[uhn-stay-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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Synonym Usage

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

See Examples For:

unstable

"The remaining resource configuration is considered unstable," the company said, warning that technical support communications could also be disrupted as teams assessed the damage.

From Barron's ● Oct. 11, 2026

RNA is relatively unstable, raising the issue of how it could have survived under harsh prebiotic conditions.

From Science Daily ● Oct. 2, 2026

Last week, the school issued a more-than-200-page internal report confronting what it called an unprecedented crisis in higher education, thanks to unstable federal funding, falling public trust and the rise of artificial intelligence.

From The Wall Street Journal ● Sep. 30, 2026

Massive flooding left a government building in western Nepal teetering on the edge of an unstable ledge before it fell into the Daram River.

From BBC ● Sep. 30, 2026

But if the building was too unstable to stand from all the bombs and the fire, how long until something else collapsed?

From "Time Bomb" by Joelle Charbonneau

Related Words

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