clinical depression I definition. 'clinical depression is the presence of depressive symptoms that rise to the level of major depressive disorder, a mental illness. Clinical depression defines the state in which the depression symptoms must be treated by a doctor. The causes of clinical depression are not specifically defined. However, as with the causes of depression in general, the causes of clinical depression are thought to be a combination of genetic, biological and environmental factors. ' ----------------------------------------- written by: the writer written in: 2019.All Rights Reserved