But sometimes depression doesn't lift after a few hours or a few days. Instead it lasts, and it can seem too heavy to bear. When someone has a depressed or sad mood that is intense and lingers almost all day, almost every day for 2 weeks or more, it may be a sign that the person has developed major depression. Major depression, sometimes called clinical depression, is beyond a passing depressed mood - it is the term mental health professionals use for depression that has become an illness in need of treatment. Another form of serious depression is called bipolar disorder, which includes extreme low moods (major depression) as well as extreme high moods (these are called manic episodes).