Not stable; not firm, fixed, or constant; subject to change or overthrow.
- Tending strongly to change: unstable weather.
- Not constant; fluctuating: unstable vital signs.
- Lacking control of one's emotions; marked by unpredictable behavior.
- Not firmly placed; unsteady: an unstable ladder.
- Chemistry Decomposing readily.
- Chemistry Highly or violently reactive.
- Physics Decaying with relatively short lifetime. Used of subatomic particles.
- Physics Radioactive.