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Major depressive disorder, also known as major depression, unipolar depression, clinical depression, or simply depression, is a mental disorder characterized by a pervasive low mood, loss of interest in usual activities and diminished ability to experience pleasure. The diagnosis is made if a person has suffered one or more major depressive episodes. The onset is... usually in early- to mid-adulthood. Diagnosis is based on the patient's self-reported experiences and observed behavior. There is no laboratory test for major depression, although physicians often test for physical conditions that may cause similar symptoms before arriving at a diagnosis. The course varies widely: it can be a once-in-a-lifetime event or have multiple recurrences; it can appear either gradually or suddenly; and can either last for a few months or be a life-long disorder.

The term "depression" is commonly used in the vernacular to describe a temporary depressed mood, when a person may feel sad or "down". Ideas about what causes and constitutes depression have evolved over the centuries. Today, mental health professionals regard chronic and severe depression as a serious and often disabling condition that can significantly affect a person's work, family and school life, sleeping and eating habits, general health and ability to enjoy life.[1] Depression is a major risk factor for suicide; in addition, people with depression suffer from higher mortality from other causes.[2] When specific treatment is indicated, it usually consists of psychotherapy and antidepressants.
Source : Wikipedia, the free encyclopedia