"having two poles;" see bi- "two" + polar. From 1810 in figurative sense of "of double aspect;" 1859 with reference to anatomy ("having two processes from opposite poles," of nerve cells). 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).
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