"having two poles," from bi- + polar; 1810 with figurative sense of "of double aspect;" 1859 with reference to physiology. Psychiatric use in reference to what had been called manic-depressive psychosis is said to have begun 1957 with German psychiatrist Karl Leonhard. The term became popular early 1990s. Bipolar disorder was in DSM III (1980).
bipolar bi·po·lar (bī-pō'lər)
Having two poles; used especially of nerve cells in which the branches project from two usually opposite points.
Of or relating to both ends or poles of a bacterial or other cell.
Of or relating to a major affective disorder that is characterized by episodes of mania and depression.