Bipolar disorder (previously known as manic depression) is a diagnostic category describing a class of mood disorders in which the person experiences states or episodes of depression and/or mania, hypomania, and/or mixed states. Left untreated, it is a severely disabling psychiatric condition. The difference between bipolar disorder and unipolar disorder (also called major depression) — for the purpose of this definition — is that bipolar disorder involves "energized" or "activated" mood states in addition to depressed mood states. The duration and intensity of mood states varies widely among people with the illness.
Fluctuating from one mood state to another is called "cycling" or having mood swings. Mood swings cause impairment not only in one's mood, but also in one's energy level, sleep pattern, activity level, social rhythms and thinking abilities. Many people become fully disabled — for significant periods of time — and during this time have great difficulty functioning.