unstable

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Related to Instable: subtle, unstable, exasperating

unstable

1. (of a chemical compound) readily decomposing
2. Physics
a. (of an elementary particle) having a very short lifetime
b. spontaneously decomposing by nuclear decay; radioactive
3. Electronics (of an electrical circuit, mechanical body, etc.) having a tendency to self-oscillation
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

unstable

[¦ən′stā·bəl]
(physics)
Capable of undergoing spontaneous change, as in a radioactive nuclide or an excited nuclear system.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.