In the fields of psychology and psychiatry, the terms depression or depressed refer to both expected and pathologically chronic or severe levels of sadness, perceived helplessness, disinterest, and other related emotions and behaviours. The Diagnostic and Statistical Manual of Mental Disorders (DSM) states that a depressed mood is often reported as being: "depressed, sad, hopeless, discouraged, or 'down in the dumps'." In traditional colloquy, "depressed" is often synonymous with "sad," but both clinical and non-clinical depression can also refer to a conglomeration of more than one feeling. Such a mixture can include (but is not limited to) anger, fear, anxiety, despair, guilt, apathy, and/or grief, in addition to what many people would describe as typical "sadness."

Major depressive disorder, also known as major depression, unipolar depression, unipolar disorder, clinical depression, or simply depression, is a mental disorder characterized by a pervasive low mood and loss of interest or pleasure in usual activities. The diagnosis is made if a person has suffered one or more major depressive episodes, and 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 of the disorder varies widely, from a one-off occurrence to a lifelong disorder with recurrent episodes. The most common time of onset is between the ages of 30 and 40, with a later peak between 50 and 60. Major depression appears significantly more often in women than men.

Both psychological and biological causes have been proposed, and the question of whether there are two separate conditions or a continuum of a single disorder has been hotly debated since the 1920s. Current classification has favored the latter theory since the creation of the term major depressive disorder in 1980. The neurotransmitters serotonin and norepinephrine have been implicated, and most antidepressants work to increase their active levels in the brain. However, the relief of symptoms usually occurs several weeks or more after changes in neurotransmitter levels, which suggests that the precise role of neurotransmitter levels in depressive illness is still murky. A host of psychological factors has also been implicated, and various forms of psychotherapy are used to address them. Hospitalization may be necessary in cases associated with self-neglect or a significant risk of suicide, and electroconvulsive therapy is sometimes used in severe cases.

The term depression is commonly used 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. By contrast, major depression can be a serious and often disabling condition that can significantly affect a person's work, family and school life, sleeping and eating habits, and general health. However, authorities such as Australian psychiatrist Gordon Parker have proposed it may be overdiagnosed, and current diagnostic standards arguably have the effect of medicalizing sadness or misery.

Around 3.4 percent of people with major depression commit suicide. Up to 60 percent of all people who commit suicide have a mood disorder, such as depression, and their risk may be especially high if they feel a marked sense of hopelessness or have both depression and borderline personality disorder. Depressed people have a shortened life expectancy, being more susceptible to and likely to die from conditions such as heart disease than the non-depressed.