From The Art and Popular Culture Encyclopedia
"Alton, a clerk in England [...] did not show the slightest trace of emotion, and gave no explanation of the motive or circumstances of his horrible deed. He was a psychopathic individual, and occasionally subject to fits of depression with taedium vitae. His father had had an attack of acute mania. A near relative suffered from mania with homicidal impulses. A. was executed."--Psychopathia Sexualis (1886) by Richard Freiherr von Krafft-Ebing
In everyday language depression refers to any downturn in mood, which may be relatively transitory and perhaps due to something trivial. This is differentiated from Clinical depression which is marked by symptoms that last two weeks or more and are so severe that they interfere with daily living.
In the field of psychiatry the word depression can also have this meaning but more specifically refers to a mental illness when it has reached a severity and duration to warrant a diagnosis. 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'."
Illnesses featuring depression
A number of psychiatric syndromes feature depressed mood as a main symptom. Mood disorders are a group of disorders considered to be primary disturbances of mood. Within them, major depressive disorder (MDD), commonly called "major depression" or "clinical depression", is a condition where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all daily activities. People suffering bipolar disorder may also experience major depressive episodes. Dysthymia is a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Outside the mood disorders, moderate chronic depressed mood is also commonly a feature of borderline personality disorder. Adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode.
Depressed mood can be the result of a number of infectious diseases and physiological problems. For example, mononucleosis (glandular fever), which can be caused by two different viral infections, often results in symptoms that mimic a depressive psychiatric disorder; and depression is often one of the early symptoms of hypothyroidism (reduced activity of the thyroid gland). For a discussion of non-psychiatric medical illnesses that can cause depressed mood, see Depression (differential diagnoses).