A mood disorder is the term given for a group of diagnoses in the DSM IV TR classification system where a disturbance in the person's emotional mood is hypothesised to be the main underlying feature.[1] The classification is known as mood (affective) disorders in ICD 10.

English psychiatrist Henry Maudsley proposed an overarching category of affective disorder.[2] The term was then replaced by mood disorder, as the latter term refers to the underlying or longitudinal emotional state, whereas the former refers to the external expression observed by others.[1]

Two groups of mood disorders are broadly recognized; the division is based on whether the person has ever had a manic or hypomanic episode. Thus, there are depressive disorders, of which the best known and most researched is major depressive disorder commonly called clinical depression or major depression, and bipolar disorder, formerly known as "manic depression" and described by intermittent periods of manic and depressed episodes.
Bipolar disorders

* Bipolar disorder, a mood disorder formerly known as "manic depression" and described by alternating periods of mania and depression (and in some cases rapid cycling, mixed states, and psychotic symptoms). Subtypes include:

* Bipolar I is distinguished by the presence or history of one or more manic episodes or mixed episodes with or without major depressive episodes. A depressive episode is not required for the diagnosis of Bipolar I disorder, but depressive episodes are often part of the course of the illness.
* Bipolar II consisting of recurrent intermittent hypomanic and depressive episodes.
* Cyclothymia is a milder form of bipolar disorder, consisting of recurrent hypomanic and dysthymic episodes, but no full manic episodes or full major depressive episodes.
* Bipolar Disorder NOS, sometimes called "sub-threshold" bipolar, indicates that the patient suffers from some symptoms in the bipolar spectrum (e.g. manic and depressive symptoms) but does not fully qualify for any of the three formal bipolar DSM-IV diagnoses mentioned above.

It is estimated that roughly one percent of the adult population suffers from Bipolar I, roughly one percent of the adult population suffers from Bipolar II or Cyclothymia, and somewhere between two and five percent suffer from "sub-threshold" forms of bipolar disorder.