What does "unstable" mean?

1. (physically shaky) likely to fall, move, or collapse because it is not firm. 2. (likely to change suddenly) changing often and without warning, like the weather or a country's politics. 3. (emotionally volatile) having moods or feelings that change very quickly or in a worrying way. 4. (chemically reactive) likely to change its form or explode easily. 5. (radioactive) having a nucleus that changes and releases energy. 6. (to release from a stable) to let an animal, like a horse, out of its building.

unstable

adj. · v.

US //ənˈsteɪbəɫ// UK //ʌnstˈeɪbəl// un·sta·ble

1 physically shaky (adj.)
B2 Upper Intermediate

likely to fall, move, or collapse because it is not firm.

lacking physical stability or equilibrium; precarious in position or structure.

Example

Don't climb that ladder; it looks very unstable on this uneven ground.

Example

The engineers warned that the bridge's foundations were becoming unstable due to the constant erosion of the riverbed.
2 likely to change suddenly (adj.)
B2 Upper Intermediate

changing often and without warning, like the weather or a country's politics.

prone to sudden or unpredictable change; lacking a constant or reliable state.

Example

The political situation in the region remains unstable after the recent elections.

Example

Economists are concerned that the unstable market conditions will discourage long-term investment in the manufacturing sector.

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3 emotionally volatile (adj.)
B2 Upper Intermediate

having moods or feelings that change very quickly or in a worrying way.

characterised by emotional or mental volatility; prone to sudden shifts in temperament.

Example

He has been emotionally unstable since he lost his job last month.

Example

The witness was deemed unreliable due to her unstable mental state at the time of the incident.
- volatile
- unbalanced
- balanced
- sane
4 chemically reactive (adj.)
C1 Advanced Technical Science

likely to change its form or explode easily.

readily undergoing chemical decomposition or change; highly reactive.

Example

The scientist handled the unstable liquid with great care in the lab.

Example

Certain chemical compounds become highly unstable when exposed to direct sunlight or sudden changes in temperature.
- reactive
- volatile
- inert
- stable
5 radioactive (adj.)
C1 Advanced Technical Science

having a nucleus that changes and releases energy.

relating to an atomic nucleus that is prone to spontaneous radioactive decay.

Example

Unstable isotopes break down over time and release radiation.

Example

The laboratory specialises in measuring the decay rates of unstable atomic nuclei produced during fission.
- radioactive
- stable
6 to release from a stable (v.)
C2 Proficiency Archaic

to let an animal, like a horse, out of its building.

to release an animal from a stable. Transitive use.

Example

The groom went to unstable the horses for the morning ride.

Example

At the first sign of fire, the stable hands rushed to unstable the panicked stallions and lead them to the safety of the paddock.
- release
- free
- stable

Etymology 1

From Middle English unstable; equivalent to un- (not) + stable.

Etymology 2

From un- (reversive prefix) + stable.