A major depressive episode (MDE) is a period characterized by symptoms of major depressive disorder (MDD). Those affected primarily exhibit a depressed mood for at least two weeks or more, and a loss of interest or pleasure in everyday activities. Other symptoms can include feelings of emptiness, hopelessness, anxiety, worthlessness, guilt, irritability, changes in appetite, difficulties in concentration, difficulties remembering details, making decisions, and thoughts of suicide. Insomnia or hypersomnia and aches, pains, or digestive problems that are resistant to treatment may also be present.

Although the exact origin of depression is unclear, it is believed to involve biological, psychological, and social aspects. Socioeconomic status, life experience, genetics, and personality traits are believed to be factors in the development of depression and may represent an increased risk of developing a major depressive episode.

In the 19th century, the term "depression" was first used as "mental depression", suggesting depression as essentially a mood or affect disorder. In modern times, depression, more often severe cases, is more noted as an absence of pleasure, with feelings of emptiness and flatness.

In the United States and Canada, the costs associated with major depression are comparable to those related to heart disease, diabetes, and back problems and are greater than the costs of hypertension. According to the Nordic Journal of Psychiatry, there is a direct correlation between a major depressive episode and unemployment.

Treatments for a major depressive episode include psychotherapy and antidepressants. Electroconvulsive therapy and transcranial magnetic stimulation may be used in cases that do not respond to other treatments. Recovery chance is higher when medication and therapy are combined in moderate-to-severe cases, although in more severe cases, hospitalization or intensive outpatient treatment may be required.

Depression can produce more disability than war, HIV/AIDS, and cancer combined. It is the leading cause of disability in the world; according to WHO (see here).

Depressed mood is the most common symptom of a major depressive episode. Either a depressive mood or a loss of interest or pleasure in everyday activities for most of the day or every day must be present for a diagnosis of a major depressive episode. In addition, the person may experience one or more of the following emotions: sadness, emptiness, hopelessness, indifference, anxiety, tearfulness, pessimism, emotional numbness, or irritability. In children and adolescents, a depressed mood often presents as irritability. Withdrawal from social settings and neglect of personal relationships often accompany depressed mood, and may be noticed by those close to the person.

Major depressive episodes are known to cause sleep disturbances such as insomnia or, less frequently, hypersomnia. Symptoms of insomnia include trouble falling asleep, trouble staying asleep, or waking up too early in the morning. Hypersomnia may include sleeping for prolonged periods at night or increased sleeping during the daytime. Sleep may not be restful, and the person may feel sluggish despite many hours of sleep, which may be a factor in the worsening of their depressive symptoms, which interfere with other aspects of their lives. This type of sleep disorder may make it harder to fall and stay asleep at night than during the day. Hypersomnia is often associated with atypical depression as well as seasonal affective disorder.