unstable

UK: /ʌnˈsteɪb^əl/ US: /ʌnˈsteɪbəl/ ,(un stā′bəl)

WordReference Random House Learner's Dictionary of American English © 2025

un•sta•ble /ʌnˈsteɪbəl/   adj. 

1. not stable;
   unsteady:
   an unstable foundation.
2. liable to change or fluctuate quickly;
   irregular:
   an unstable heartbeat.
3. showing an inability to keep one's emotions stable or under some control:
   mentally unstable.
4. Chemistry noting chemical compounds that readily change into other compounds.

See -stab-.

WordReference Random House Unabridged Dictionary of American English © 2025

un•sta•ble  (un stā′bəl), adj. 

1. not stable;
   not firm or firmly fixed;
   unsteady.
2. liable to fall or sway.
3. unsteadfast;
   inconstant;
   wavering:
   unstable convictions.
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.

• 1175–1225; Middle English; see un-¹, stable²

un•sta′ble•ness, n. 
un•sta′bly, adv. 
Synonyms

• 2. precarious.
• 2, 3. See unsettled. 
• 3. vacillating.

Collins Concise English Dictionary © HarperCollins Publishers::

unstable / ʌnˈsteɪb^əl/ adjective

1. lacking stability, fixity, or firmness
2. disposed to temperamental, emotional, or psychological variability
3. (of a chemical compound) readily decomposing
4. (of an elementary particle) having a very short lifetime
5. spontaneously decomposing by nuclear decay; radioactive: an unstable nuclide
6. (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation

unˈstableness noun

unˈstably adverb