- 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.
- Chemistry. noting compounds that readily decompose or change into other compounds.
- 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; radioactivean unstable nuclide
- electronics (of an electrical circuit, mechanical body, etc) having a tendency to self-oscillation
adj.early 13c., “apt to move,” from un- (1) “not” + stable (adj.). Cf. Middle High German unstabel. Meaning “liable to fall” is recorded from c.1300; sense of “fickle” is attested from late 13c. An Old English word for this was feallendlic, which might have become *fally.
- Liable to undergo spontaneous decay into some other form. For example, the nucleus of uranium 238 atom is unstable and changes by radioactive decay into the nucleus of thorium 234, a lighter element. Many subatomic particles, such as muons and neutrons, are unstable and decay quickly into other particles. See more at decay.
- Relating to a chemical compound that readily decomposes or changes into other compounds or into elements.
- Relating to an atom or chemical element that is likely to share electrons; reactive.
- Characterized by uncertain or inadequate response to treatment and the potential for unfavorable outcome, as the status of a medical condition or disease.