Longman Dictionary of Contemporary English homepage

unstable

 adjective
     
Related topics: Chemistry
un‧sta‧ble
1 likely to change suddenly and become worse [↪ instability]:
The political situation is still very unstable.
an unstable relationship
2 something that is unstable is likely to move or fall
3 someone who is unstable changes very suddenly so that you do not know how they will react or behave:
a mentally unstable man
4HC an unstable chemical is likely to separate into simpler substances
Definition from the Longman Dictionary of Contemporary English
Advanced Learner's Dictionary.

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