Depression (clinical depression or major depressive disorder) is a serious mood disorder that causes persistent feelings of sadness and a loss of interest in activities once enjoyed. It is not a sign of weakness or something a person can simply "snap out of". Key Characteristics

: Unlike occasional sadness, depressive symptoms must persist for at least two weeks to be diagnosed.

: Changes in appetite or weight, chronic fatigue, sleep disturbances (insomnia or oversleeping), and unexplained aches or pains.