any of a group of mood disorders in which symptoms of mania and depression alternate. In DSM–IV–TR and DSM–5, the group includes primarily the following subtypes: bipolar I disorder, in which the individual fluctuates between episodes of mania or hypomania and major depressive episodes or experiences a mix of these: bipolar II disorder, in which the individual fluctuates between major depressive and hypomanic episodes; and cyclothymic disorder. 

The former official name for bipolar disorders, manic-depressive illness, is still in frequent use.