According to the American Psychiatric Association DSM-IV criteria,<unk>  is not regarded as a separate disorder. It is called a "course specifier" and may be applied as an added description to the pattern of major depressive episodes in patients with major depressive disorder or patients with bipolar disorder. The<unk>  Pattern Specifier" must meet four criteria: depressive episodes at a particular time of the year; remissions or mania/hypomania at a characteristic time of year; these patterns must have lasted two years with no nonseasonal major depressive episodes during that same period; and these<unk>  depressive episodes outnumber other depressive episodes throughout the patient's lifetime. The Mayo Clinic describes three types of<unk> , each with its own set of symptoms.
Seasonal affective disorder
Major depressive episode
Major depressive disorder
Depression 
Late life depression
Premenstrual syndrome
Secondary mania
Postpartum depression