Mood disorders are a broad category of psychopathology that includes depressive disorders and bipolar disorders. These conditions are defined in terms of episodes in which the person's behavior is dominated by either clinical depression or mania. Depression, or unipolar mood disorder, refers either to a symptom, a mood, or a clinical syndrome. The first widely accepted classification system to include depression was proposed by Emil Kraeplin. Mental disorders were divided into two categories by Kraeplin: dementia praecox and manic-depressive psychosis. His differentiation was based on such salient features as age of onset, clinical symptoms, and courses of the disorder. According to Kraeplin, manic-depressive psychosis in his patients was usually characterized by an episodic, recurrent course, with a relatively good prognosis. The psychiatrists Adolf Meyer and Sigmund Freud also greatly influenced to the classification system of depressive disorders. Meyer's works seemed to stress the biological and psychological functions that play a role when adapting to one's environment, thus classifying depression as a reaction to the environment rather than an illness.