An emotional state or mood characterized by one or more of these symptoms: sad mood, low energy, poor concentration, sleep or appetite changes, feelings of worthlessness or hopelessness, and thoughts of suicide.

Depression may signify a mood, a symptom, or a syndrome. As a mood, it refers to temporary feelings of sadness, despair, and discouragement. As a symptom, it refers to these feelings when they persist and are associated with such problems as decreased pleasure, hopelessness, guilt, and disrupted sleeping and eating patterns. The entire syndrome is also referred to collectively as a depression or depressive disorder. At any given time about 10 percent of all people suffer some of the symptoms of depression at an economic cost of more than $30 billion annually...