Definition of unstable adjective from the Oxford Advanced American Dictionary

      

    unstable

     adjective
    adjective
    NAmE//ʌnˈsteɪbl//
     
     
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  1. 1likely to change suddenly synonym volatile The political situation remains highly unstable.
  2. 2if people are unstable, their behavior and emotions change often and suddenly because their minds are upset Thesaurusmentally illinsane depressed neurotic psychotic disturbed unstableThese words all describe someone who is suffering from a mental illness.mentally ill suffering from an illness of the mind, especially in a way that affects the way you think and behaveinsane [not usually before noun] (somewhat formal) suffering from a serious mental illness and unable to live in normal society:The question is, was the woman insane when she committed the crime? In informal English, insane can describe someone who is not suffering from a mental illness but whose mind does not work normally, especially because they are under pressure. This meaning is used especially in the phrases go insane and drive somebody insane.depressed (medical) suffering from a medical condition in which a person feels very sad and anxious over a long period of time, and often has physical symptoms such as being unable to sleep:He was diagnosed as clinically depressed. In informal English, depressed is sometimes used to describe someone who is just feeling very sad, but in correct medical usage it only describes people who feel very sad for weeks at a time.neurotic (medical) suffering from or connected with neurosis (= a mental illness in which a person suffers strong feelings of fear and worry):the treatment of anxiety in neurotic patients In informal English neurotic is also used to describe someone who is not suffering from a mental illness, but is not behaving in a calm way because they are worried about something:He became neurotic about keeping the house clean.psychotic (medical) suffering from or connected with psychosis (a serious mental illness in which thought and emotions lose connection with external reality). In informal English, psychotic is sometimes used to describe anyone suffering from a mental illness, but in correct medical usage it only describes people who have difficulty relating to external reality. It contrasts with neurotic which describes people who are less seriously mentally ill and are still able to distinguish what is real from what is not.disturbed mentally ill, especially because of very unhappy or shocking experiences:He works with emotionally disturbed children.unstable having emotions and behavior that are likely to change suddenly and unexpectedlyPatterns neurotic/psychotic/disturbed/unstable behavior neurotic/psychotic illnesses/disorders/symptoms/patients seriously mentally ill/depressed/neurotic/psychotic/disturbed emotionally/mentally disturbed/unstable
  3. 3likely to move or fall Icebergs are notoriously unstable and are likely to turn over.
  4. 4(technology) (of a substance) not staying in the same chemical or atomic state chemically unstable opposite stable see also instability
See the Oxford Advanced Learner's Dictionary entry: unstable