Definition for Major depressive disorder
From Biology Forums Dictionary
1) 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.
2) Major depressive disorder (unipolar depression) is diagnosed when someone consistently experiences five or more depressive symptoms, including either depressed mood or loss of interest or pleasure, for at least two weeks straight.