un·sta·ble (ŭn-stā'bəl) adj.
  1. Fickle.
  2. Lacking control of one's emotions; marked by unpredictable behavior.
  3. (Chemistry) Decomposing readily.
  4. (Chemistry) Highly or violently reactive.
  5. (Physics) Decaying with relatively short lifetime. Used of subatomic particles.
  6. (Physics) Radioactive.