Quick changes of emotions or unstable mood is rather common for everyone. So this symptom can have very different forms and severity. Mild forms are not at all a sign of a major psychiatric disorders.
But we refer to people with a quickly changing emotional response who have rapid and extreme mood changes within a very short period, and without appropriate external reasons. Usually this lasts only for some minutes or hours (in contrast to longer periods of depression or mania in bipolar disorders).
The most prominent forms of this problem are common among patients with Borderline Personality Disorder More. Other examples of affective lability can be found in women with Premenstrual Dysphoric Disorder. More.
It is also common among children, adolescents and adults with ADHD.
Longer lasting periods of inappropriate affective reactions are also typical for organic brain disorders and severe psychiatric disorders like schizophrenia.