Major Depressive Disorder (MDD), is characterized by a severely depressed mood that persists for at least two weeks. Major Depressive Disorder is specified as either "a single episode" or "recurrent"; periods of depression may occur as discrete events or recur over the lifespan. Episodes of major or clinical depression may be further divided into mild, major or severe. Where the patient has already had an episode of mania or markedly elevated mood, a diagnosis of bipolar disorder (also called bipolar affective disorder) is usually made instead of MDD; depression without periods of elation or mania is therefore sometimes referred to as unipolar depression because the mood remains on one pole. The diagnosis also usually excludes cases where the symptoms are a normal result of bereavement. Diagnosticians recognize several possible subtypes of Major Depressive Disorder. Depression with Melancholic Features - Melancholia is characterized by a loss of pleasure (anhedonia) in most or all activities, a failure of reactivity to pleasurable stimuli, a quality of depressed mood more pronounced than that of grief or loss, a worsening of symptoms in the morning hours, early morning waking, psychomotor retardation, anorexia (excessive weight loss, not to be confused with Anorexia Nervosa), or excessive guilt. Depression with Atypical Features - Atypical Depression is characterized by mood reactivity (paradoxical anhedonia) and positivity, significant weight gain or increased appetite, excessive sleep or somnolence (hypersomnia), leaden paralysis, or significant social impairment as a consequence of hypersensitivity to perceived interpersonal rejection. Contrary to its name, atypical depression is the most common form of depression. Depression with Psychotic Features - Some people with Major Depressive or Manic episode may experience psychotic features. They may be presented with hallucinations or delusions that are either mood-congruent (content coincident with depressive themes) or non-mood-congruent (content not coincident with depressive themes). It is clinically more common to encounter a delusional system as an adjunct to depression than to encounter hallucinations, whether visual or auditory.