Earliest medical sources show an awareness that "excitement" and "depression" may have represented to aspects of a single disorder. Arteus, in the second century A.D., discussed "mania" as he described a group of euphoric patients who would "Laugh, play, dance night and day, and sometimes go openly to the market crowned, as day, as if victors in some contest of skill", only later to appear "torpid, dull and sorrowful".  In 1686 Theophile Bonet coined the term "manico-melancolicus," to denote the connections between mania and melancholia.

It was not until the 1830's that two French physicians, Falret and Baillarger become the first to isolate and identify the symptoms of the disease Falret caller "Circular Insanity".  It is remarkable how Falret's description of symptoms and hereditary factors are so similar to descriptions found in present day books and journals.  Falret even encouraged physicians to diversify medications used in the treatment of manic-depressive  illness in the hopes that one of them might one day discover an effective drug therapy.  It is to Emil Kraepeling (Germany 1856-1926)  that the most credit goes for his painstakingly accurate and vivid descriptions of manic-depressive illness (1904).

A manic-depressive illness or "bipolar mood disorder" is a disturbance of a person's mood characteristics by alternating periods of depression and manic.  Switching from one mood to another is referred to as a mood swing.  Mood swings can be mild, moderate or severe and are accompanied by changes in thinking and behavior.  The course of the illness varied from patient to patient.  Without treatment, the frequency and severity of this recurring illness can increase over the years.

Although lay people will ordinarily use the term "manic-depressive illness", psychiatric professionals are increasing using the term "bipolar mood disorder" as is demanded by current diagnostic manuals.  Bipolar refers to the "two poles" of the continuum of mood or "spirits" with depression or feeling down at one end and mania or feeling high at the other end.