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.