Depression is not just a form of 'extreme sadness,' it is a disorder that affects the brain and the body, affecting the consciousness, thinking, and behavior in the mind, and the immune system and peripheral nervous system in the body. The word 'disorder' is included in the definition of 'depressive disorder' because it signifies that the person becomes unable to function in terms of personal and work responsibilities. The medical definition of depression is a sustained abnormality in a person's mood or feelings of despair, hopelessness, and self-hatred. Unlike a passing sad mood, depression is called a disorder because it interferes with ones normal functioning in everyday life. The formal diagnostic criteria are described in the DSM-IV .
Depression can occur at any age, though the average onset is during the second decade of life.
Different people can have very different courses of depression. Most commonly, people with untreated depression experience episodes of depression that come and go, increasing in frequency with age.