WordWeb Online

1. Highly or violently reactive
   "sensitive and highly unstable compounds"
    
2. Affording no ease or reassurance
   - precarious
    
3. Suffering from severe mental illness
   "of unstable mind";
   - mentally ill, unsound
    
4. Disposed to psychological variability
   "his rather unstable religious convictions"
    
5. Subject to change; variable
   "everything was unstable following the coup";
   - fluid
    
6. Lacking stability, fixity or firmness
   "unstable political conditions"; "the tower proved to be unstable in the high wind"; "an unstable world economy"

Derived forms: unstabler, unstablest

See also: changeable, changeful, coseismal, coseismic, crank, cranky [informal], explosive, impermanent, inconstant, insane, irresolute, reactive, rickety, rocky, seismal, seismic, shaky, temporary, tender, tippy, tipsy, top-heavy, tottering, uneasy, unsteady, volatile, volcanic, wobbly, wonky [informal]

Antonym: stable

Encyclopedia: Unstable