Clinical depression (also called major-depressive disorder or unipolar depression) is a common psychiatric disorder, characterized by a pervasive low mood, loss of interest in usual activities and diminished ability to experience pleasure.

Although the term "depression" is commonly used to describe a temporary depressed mood when one "feels blue", clinical depression is 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. The course of clinical depression varies widely: depression can be a once in a life-time event or have multiple recurrences, it can appear either gradually or suddenly, and either last for a few months or be a life-long disorder. Having depression is a major risk factor for suicide; in addition, people with depression suffer from higher mortality from other causes.

Clinical depression may be isolated or be a secondary result of a primary condition such as bipolar disorder or chronic pain. When specific treatment is indicated, this is usually psychotherapy and/or antidepressants.