A mental health disorder characterized by persistently depressed mood or loss of interest in activities, causing significant impairment in daily life. Possible causes include a combination of biological, psychological, and social sources of distress. Increasingly, research suggests these factors may cause changes in brain function, including altered activity of certain neural circuits in the brain.

The persistent feeling of sadness or loss of interest that characterizes major depression can lead to a range of behavioral and physical symptoms. These may include changes in sleep, appetite, energy level, concentration, daily behavior, or self-esteem. Depression can also be associated with thoughts of suicide.