Depression is a state of low mood and aversion to activity that can affect a person’s thoughts, behaviour, feelings and physical well-being. Depressed people may feel sad, anxious, empty, hopeless, helpless, worthless, guilty, irritable, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, or problems concentrating, remembering details or making decisions; and may contemplate or attempt suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains or digestive problems that are resistant to treatment may be present.
Depressed mood is a normal reaction to certain life events, a symptom of some medical conditions (e.g., Addison’s disease, hypothyroidism) and treatments (e.g., hepatitis C drug therapy), and a feature of certain psychiatric syndromes.
Illnesses featuring depression
A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD), commonly called major depression or clinical depression, where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated energy levels, cognition and mood, but may also involve one or more depressive episodes.
Outside the mood disorders, borderline personality disorder commonly features depressed mood, and 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, which can be caused by two different viral infections (Epstein-Barr and cytomegalo virus), 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). Disturbed circadian rhythm may contribute to depression. For a discussion of non-psychiatric conditions that can cause depressed mood.
Source and References http://en.wikipedia.org/wiki/Depression_(mood)