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.

---

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

---

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.

---

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

Part yoga, part Pilates and entirely unstable: Inside the high-intensity, low-impact pool workout making waves at the Annenberg Community Beach House.

From Los Angeles Times ● Jul. 28, 2026

If the fire is powerful enough and the atmosphere is unstable, that cloud can grow into a towering cumulonimbus - the same type of cloud associated with thunderstorms.

From BBC ● Jul. 27, 2026

Over time, portions of that gas cool, become unstable, and collapse into thin filaments.

From Science Daily ● Jul. 18, 2026

"There are many unstable and uncertain external factors, and the domestic contradiction of strong supply and weak demand is prominent. The foundation for the economy to improve still needs to be consolidated," it added.

From Barron's ● Jul. 15, 2026

And here was the nimble Army Corps of Engineers planning to hold back 250 billion gallons of water with notoriously unstable glacial till.

From "A Walk in the Woods" by Bill Bryson

Related Words

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