Major depressive disorder (MDD), also known as clinical depression or simply depression, is a mental disorder characterized by the presence of a severely low mood that persists for at least two weeks and which pervades across most, if not all, aspects of life.
The onset of MDD can occur in children and adolescents and in younger and older adults. MDD most commonly occurs between 20 and 40 years of age, and women are twice as likely as men to be diagnosed with it. Lifetime rates of MDD are higher in the developed world (15%) compared to the developing world (11%). According to the Global Burden of Disease study, MDD causes the second-most years lived with a disability, after lower back pain.