Major depressive disorder (MDD), also known as clinical depression, is a mental health condition characterized by at least two weeks of a pervasive low mood, low self-esteem, and loss of interest or pleasure in activities that are normally enjoyable. It is a serious mood disorder that can affect how a person feels, thinks, and behaves, leading to emotional and physical problems, and difficulty in daily functioning. The diagnosis of MDD involves experiencing five or more specific symptoms, including either depressed mood or anhedonia (loss of interest or pleasure), lasting for more than two weeks and causing significant impairment in social, occupational, or other important areas of functioning. Other symptoms may include feelings of guilt or worthlessness, changes in appetite or sleep, lack of energy, difficulty concentrating, psychomotor agitation or retardation, and suicidal thoughts. MDD is believed to be caused by a combination of genetic, environmental, and psychological factors. It typically manifests in episodes, which can be recurrent, and it is treatable through psychotherapy, medications, and in some cases, electroconvulsive therapy. In summary, MDD is a common and serious mental disorder marked by prolonged and severe depressive episodes that interfere with a person's ability to function daily. It is different from general depression in severity and treatment approach. This information is drawn from authoritative medical and clinical sources. If you need detailed diagnostic criteria, treatment options, or subtype information, let me know!