According to DSM-IV, the diagnostic manual from American Psychiatric Association, criteria used by mental health professionals, you have Major Depressive Disorder if:

You have had an episode of depression lasting at least two weeks with at least five of the following symptoms:
(1) You are depressed, sad, blue, tearful.
(2) You have lost interest or pleasure in things you previously liked to do.
(3) Your appetite is much less or much greater than usual and you have lost or gained weight.
(4) You have a lot of trouble sleeping or sleep too much.
(5) You are so agitated, restless, or slowed down that others have begun to notice.
(6) You are tired and have no energy.
(7) You feel worthless or excessively guilty about things you have done or not done.
(8) You have trouble concentrating, thinking clearly, or making decisions.
(9) You feel you would be better off dead or have thoughts about killing yourself.

These symptoms are severe enough to upset your daily routine, or to seriously impair your work, or to interfere with your relationships.

The depression does not have a specific cause like alcohol, drugs, medication side effect, or physical illness.

Your depression is not just a normal reaction to the death of a loved one.