Clinical depression (also called major depressive disorder, or unipolar depression when compared to bipolar disorder) is a common mood disorder in psychology and psychiatry, in which a person's enjoyment of life and ability to function socially and in day to day matters is disrupted by intense sadness, melancholia, numbness, or despair.

Clinical depression differs from the common term depression and the everyday expression of "feeling depressed". It is diagnosed medically, and treated by therapy and possibly antidepressant drugs. There are several subtypes, some of which meet the popular perception of sadness, agitation and disruption of sleeping and eating, and others of which do not disrupt enjoyment of good things but create a highly disruptive cycle of inner paralysis and lethargy.

Clinical depression affects about 7-18% of the population on at least one occasion in their lives, before the age of 40.

Although a low mood or state of dejection that does not affect functioning is often colloquially referred to as depression, clinical depression is a clinical diagnosis and may be different from the everyday meaning of "being depressed." Many people identify the feeling of being clinically depressed as "feeling sad for no reason", or "having no motivation to do anything." A person suffering from depression may feel tired, sad, irritable, lazy, unmotivated, and apathetic. Clinical depression is generally acknowledged to be more serious than normal depressed feelings. It often leads to constant negative thinking and sometimes substance abuse or self-harm. Extreme depression can culminate in its sufferers attempting or completing suicide.

Without careful assessment, delirium can easily be confused with depression and a number of other psychiatric disorders because many of the signs and symptoms are conditions present in depression, as well as other mental illnesses including dementia and psychosis.

The different types of depression and anxiety are classified separately by the DSM-IV-TR, with the exception of hypomania, which is included in the bipolar disorder category. Despite the different categories, depression and anxiety can indeed be co-occurring (occurring together), independently (without mood congruence), or comorbid (occurring together, with overlapping symptoms, and with mood congruence). In an effort to bridge the gap between the DSM-IV-TR categories and what clinicians actually encounter, experts such as Herman Van Praag of Maastricht University have proposed ideas such as anxiety/aggression-driven depression.This idea refers to an anxiety/depression spectrum for these two disorders, which differs from the mainstream perspective of discrete diagnostic categories.

Although there is no specific diagnostic category for the comorbidity of depression and anxiety in the DSM or ICD, the National Comorbidity Survey (US) reports that 58 percent of those with major depression also suffer from lifetime anxiety. Supporting this finding, two widely accepted clinical colloquialisms include