Major depressive disorder a psychiatric disorder characterized by a pervasive low mood, loss of interest in usual activities, and a diminished ability to experience pleasure (anhedonia).
Although the term "depression" is commonly used to describe a temporary depressed mood (i.e., "feeling blue"), major depressive disorder is a serious and often disabling condition that can significantly affect a person's work, family, and school life; sleeping and eating habits; general health; and ability to enjoy life. The course of major depressive disorder varies widely: depression can be a once-in-a-lifetime event or have multiple recurrences, it can appear either gradually or suddenly, and it can either last for a few months or be a life-long disorder. Major depressive disorder is a major risk factor for suicide. In addition, people with depression suffer from higher mortality from other causes.
When specific treatment is indicated, it usually consists of psychotherapy and antidepressants.
The impact of depression on the quality of an individual's life is more than that of having a recent myocardial infarction but less than tha of having heart failure..
Clinical depression
Depression 
Post travel depression
Atypical depression
Post stroke depression
Suicide ideation
Melancholic depression
Psychotic depression