in economics, major downswing in the business cycle that is characterized by sharply reduced industrial production, widespread unemployment, serious declines or cessations of growth in construction activity, and great reductions in international trade and capital movements. Unlike minor business contractions that may occur in one country independently of business cycles in other countries, severe depressions have usually been nearly worldwide in scope. The Great Depression that began in 1929, for example, was the most widespread depression in the 20th century. Compare recession; panic.

in psychology, a mood or emotional state that is marked by feelings of low self-worth or guilt and a reduced ability to enjoy life. A person who is depressed usually experiences several of the following symptoms: feelings of sadness, hopelessness, or pessimism; lowered self-esteem and heightened self-depreciation; a decrease or loss of ability to take pleasure in ordinary activities; reduced energy and vitality; slowness of thought or action; loss of appetite; and disturbed sleep or insomnia. Depression differs from simple grief or mourning, which are appropriate emotional responses to the loss of loved persons or objects. Where there are clear grounds for a person's unhappiness, depression is considered to be present if the depressed mood is disproportionately long or severe vis-à-vis the precipitating event. A person who experiences alternating states of depression and mania (abnormal elevation of mood) or hypomania (distinct, though not necessarily abnormal, elevation of mood) is said to suffer from bipolar disorder.

Depression is probably the most common psychiatric complaint and has been described by physicians since before the time of Hippocrates, who called it melancholia. The course of the disorder is extremely variable from person to person; it may be mild or severe, acute or chronic. Untreated, depression may last an average of four months or longer. Depression is twice as prevalent in women than in men. The typical age of onset is in the 20s, but it may occur at any age.

Depression can have many causes. Unfavourable life events can increase a person's vulnerability to depression or trigger a depressive episode. Negative thoughts about oneself and the world are also important in producing and maintaining depressive symptoms. However, both psychosocial and biochemical mechanisms seem to be important causes; the chief biochemical cause appears to be the defective regulation of the release of one or more naturally occurring neurotransmitters (neurotransmitter) in the brain, particularly norepinephrine and serotonin. Reduced quantities or reduced activity of these chemicals in the brain is thought to cause the depressed mood in some sufferers.

There are three main treatments for depression. The two most important—and widespread by far—are psychotherapy and psychotropic medication (drug), specifically antidepressants (antidepressant). Psychotherapy aims to alter the patient's maladaptive cognitive and behavioral responses to stressful life events while also giving emotional support to the patient. Antidepressant medications, by contrast, directly affect the chemistry of the brain and presumably achieve their therapeutic effects by correcting the chemical dysregulation that is causing the depression. Two types of medications, tricyclic antidepressants and the more recently developed selective serotonin reuptake inhibitors (SSRIs), though chemically different, both serve to prevent the presynaptic reuptake of serotonin (and in the case of tricyclic antidepressants, norepinephrine as well). This results in the buildup or accumulation of neurotransmitters in the brain and allows them to remain in contact with the nerve cell receptors longer, thus helping to elevate the patient's mood. By contrast, the antidepressants known as monoamine oxidase inhibitors (MAO) interfere with the activity of monoamine oxidase, an enzyme that is known to be involved in the breakdown of norepinephrine and serotonin. In cases of severe depression in which therapeutic results are needed quickly, electroconvulsive therapy (shock therapy) (ECT) has sometimes proved helpful. In this procedure, a convulsion is produced by passing an electric current through the person's brain. In many cases of treatment, the best therapeutic results are obtained by using a combination of psychotherapy and antidepressant medication. See also therapeutics.