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English definition of “unstable”

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unstable

adjective  /ʌnˈsteɪ·bəl/ us  
not firm and therefore not strong, safe, or likely to last: It is a poor and politically unstable country. An unstable person suffers from sudden and extreme changes of mental state: He’s emotionally unstable – you never know how he’ll react.
(Definition of unstable from the Cambridge Academic Content Dictionary © Cambridge University Press)
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