Depression may be described as feeling sad, blue, unhappy, miserable, or down in the dumps. Most of us feel this way at one time or another for short periods.
True clinical depression is a mood disorder in which feelings of sadness, loss, anger, or frustration interfere with everyday life for weeks or longer.
Major depressive disorder
Depression 
Postpartum depression
Bipolar disorder
Seasonal affective disorder
Postnatal depression
Atypical depression
Panic disorder