Major epressive disorder, also known as major depression, unipolar depression, clinical depression, or simply depression, is a psychiatric disorder characterized by a pervasive low mood, loss of interest in a person's 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.