Bipolar disorders is classification from mood disorders. Mood is a pervasive and sustained feeling tone that is experienced internally and that influences a person's behavior and perception of the world. Affect is the external expression of mood. Mood can be normal, elevated, or depressed. Healthy persons experience a wide range of moods and have an equally large repertoire of affective expressions; they feel in control of their moods and affects.
Depression is part of phase bipolar disorders that would arise as mood disorders

Mood disorders are a group of clinical conditions characterized by a loss of that sense of control and a subjective experience of great distress. Patients with elevated mood demonstrate expansiveness, flight of ideas, decreased sleep, and grandiose ideas. Patients with depressed mood experience a loss of energy and interest, feelings of guilt, difficulty in concentrating, loss of appetite, and thoughts of death or suicide. Other signs and symptoms of mood disorders include change in activity level, cognitive abilities, speech, and vegetative functions (e.g., sleep, appetite, sexual activity, and other biological rhythms). These disorders virtually always result in impaired interpersonal, social, and occupational functioning.

It is tempting to consider disorders of mood on a continuum with normal variations in mood. Patients with mood disorders, however, often report an ineffable, but distinct, quality to their pathological state. The concept of a continuum, therefore, may represent the clinician's over identification with the pathology, thus possibly distorting his or her approach to patients with mood disorder.

Patients afflicted with only major depressive episodes are said to have major depressive disorder or unipolar depression. Patients with both manic and depressive episodes or patients with manic episodes alone are said to have bipolar disorder. The terms unipolar mania and pure mania are sometimes used for patients who are bipolar, but who do not have depressive episodes.

Three additional categories of mood disorders are hypomania, cyclothymia, and dysthymia. Hypomania is an episode of manic symptoms that does not meet the full text revision of the fourth edition of Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) criteria for manic episode. Cyclothymia and dysthymia are defined by DSM-IV-TR as disorders that represent less severe forms of bipolar disorder and major depression, respectively.

The field of psychiatry has considered major depression and bipolar disorder to be two separate disorders, particularly in the last 20 years. The possibility that bipolar disorder is actually a more severe expression of major depression has been reconsidered recently, however. Many patients given a diagnosis of a mood disorders major depressive disorder reveal, on careful examination, past episodes of manic or hypomanic behavior that have gone undetected. Many authorities see considerable continuity between recurrent depressive and bipolar disorders. This has led to widespread discussion and debate about the bipolar spectrum, which incorporates classic bipolar disorder, bipolar II, and recurrent depressions.