A major depressive episode is not a disorder in itself, but is a part of another disorder, most often major depressive disorder or bipolar disorder.

A person who suffers from a major depressive episode must either have a depressed mood or a loss of interest or pleasure in daily activities consistently for at least a 2 week period. This mood must represent a change from the person's normal mood; social, occupational, educational or other important functioning must also be negatively impaired by the change in mood. A major depressive episode is also characterized by the presence of a majority of these symptoms:

• Depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad or empty) or observation made by others (e.g., appears tearful). (In children and adolescents, this may be characterized as an irritable mood.)
• Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day.
• Significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day.
• Insomnia or hypersomnia nearly every day.
• Psychomotor agitation or retardation nearly every day.
• Fatigue or loss of energy nearly every day.
• Feelings of worthlessness or excessive or inappropriate guilt nearly every day.
• Diminished ability to think or concentrate, or indecisiveness, nearly every day.
• Recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, or a suicide attempt or a specific plan for committing suicide.