Manic depression is an older term which is used informally to refer to bipolar disorder. Bipolar disorder, which is the official terminology used in the Diagnostic and Statistical Manual of Mental Disorders, is, arguably, more descriptive of the illness than manic depression, given how aptly it describes the swings from the manic pole of the illness to the depressive pole.
Bipolar disorder is actually a group of mood disorders characterized by cyclical disturbances in mood, thought and behavior. These disorders all consist of alternating periods of elevated, expansive or irritable moods, called manic episodes, and depressive episodes. They differ, however, in the severity of their phases.
Bipolar I disorder refers to patients who have had at least one episode of mania or mixed episodes (exhibiting symptoms of both depression and mania during the same period of time). Bipolar II disorder refers to patients who have had both hypomania (a milder form of mania) and major depressive episodes. Cyclothymia refers to patients who have had chronic fluctuations between hypomania and milder, subclinical depression.
A critical point in distinguishing bipolar disorder from major depressive disorder is whether the patient has had a manic episode. For a patient to be diagnosed with bipolar disorder, he must have had a manic episode lasting for at least one week or a hypomanic episode lasting for at least four days.
To learn more about bipolar disorder, please visit About.com's Guide to Bipolar Disorder.
Jacobson, James L. and Alan M. Jacobson. Psychiatric Secrets 2nd ed. Philadelphia: Hanley & Belfus, 2001.
Moore, David P. and James W. Jefferson. Handbook of Medical Psychiatry 2nd ed. Philadephia: Mosby, 2004.