Depression is different from recurring feelings of stress, anxiety, or inadequacy. It really is something different. Not only does depression cause more sustained and severe low mood, but it also interferes with the functioning in terms of work, other activities and relationships.
Physical symptoms are common.
If depression is severe enough and lasts at least two weeks, it is called major depression. Left untreated, an average episode of major depression lasts six or more months. While most individuals recover fully, some remain chronically depressed. In addition, major depression is likely to become a recurrent disorder since the majority of people have more than one episode. Less severe forms of depression such as dysthymia and minor depression may also cause personal suffering and interfere with social relationships and occupational functioning.