Depression

Clinical depression (also called major depressive disorder) is a state of intense sadness, melancholia or despair that has advanced to the point of being disruptive to an individual's social functioning and/or activities of daily living. Although a low mood or state of dejection that does not affect functioning is often colloquially referred to as depression, clinical depression is a clinical diagnosis and may be different from the everyday meaning of "being depressed". Many people identify the feeling of being depressed as "being blue", "feeling sad for no reason", or "having no motivation to do anything". Clinical depression is generally acknowledged to be more serious than normal depressed feelings.

This is an edited version of the Wikipedia article: http://en.wikipedia.org/wiki/Clinical_depression