- lacking stability, fixity, or firmness
- disposed to temperamental, emotional, or psychological variability
- (of a chemical compound) readily decomposing
- (of an elementary particle) having a very short lifetime
- spontaneously decomposing by nuclear decay; radioactive: an unstable nuclide
- (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation
WordReference Random House Learner's Dictionary of American English © 2026
un•sta•ble /ʌnˈsteɪbəl/USA pronunciation
adj.
WordReference Random House Unabridged Dictionary of American English © 2026- not stable;
unsteady:an unstable foundation. - liable to change or fluctuate quickly;
irregular:an unstable heartbeat. - showing an inability to keep one's emotions stable or under some control:mentally unstable.
- Chemistrynoting chemical compounds that readily change into other compounds.
un•sta•ble
(un stā′bəl),USA pronunciation adj.
un•sta′ble•ness, n.
un•sta′bly, adv.
- not stable;
not firm or firmly fixed;
unsteady. - liable to fall or sway.
- unsteadfast;
inconstant;
wavering:unstable convictions. - marked by emotional instability:an unstable person.
- irregular in movement:an unstable heartbeat.
- Chemistrynoting compounds that readily decompose or change into other compounds.
- 1175–1225; Middle English; see un-1, stable2
un•sta′bly, adv.
- 2. precarious. 2. 3. See unsettled. 3. vacillating.
Collins Concise English Dictionary © HarperCollins Publishers::
unstable /ʌnˈsteɪbəl/ adj
'unstable' also found in these entries (note: many are not synonyms or translations):
amyl nitrite
- astatic
- astatine
- banana republic
- chlorous acid
- chromic acid
- crisis
- dithionous acid
- flighty
- fluctuate
- fluctuant
- francium
- house of cards
- hydrogen peroxide
- hypochlorous acid
- insecure
- lifetime
- Margaret of Anjou
- mean life
- mudflow
- omega minus
- psycho
- quaking
- rocky
- slippery
- soft
- sulphurous acid
- tapper
- thiosulphuric acid
- top-heavy
- totter
- treacherous
- unsound
- untogether
- vertiginous
- biogen
- bismuthine
- brittle
- calcium cyanamide
- changeable
- chloramine
- chlorine dioxide
- crank
- critical
- cyanamide
- cyanic acid
- destabilize
- disoriented
- dithionic acid
- erratic