Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Major depressive disorder is a mental disorder characterized by an all-encompassing low mood accompanied by low self-esteem, and by loss of interest or pleasure in normally enjoyable activities. Major depressive disorder is a disabling condition that adversely affects a person's family, work or school life, sleeping and eating habits, and general health. Learn more about major depressive disorder, its possible causes, effects and treatments in the following pages.