Bipolar disorder is a mood disorder that can cause intense mood swings: Sometimes you may feel extremely "up," elated, irritable, or energized. This is called a manic episode. Other times you may feel "down," sad, indifferent, or hopeless. This is called a depressive episode. You may have both manic and depressive symptoms together. This is called a mixed episode. Along with mood swings, bipolar disorder causes changes in behavior, energy levels, and activity levels. Bipolar disorder used to be called other names, including manic depression and manic-depressive disorder.

Source: Medline Plus