Definition of Depression: Feelings of 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. But true clinical depression is a mood disorder in which feelings of sadness, loss, anger, or frustration interfere with everyday life for an extended period of time. Low or irritable mood most of the time; A loss of pleasure in usual activities; Trouble sleeping or sleeping too much; A big change in appetite, often with weight gain or loss; Tiredness and lack of energy; Feelings of worthlessness, self-hate, and guilt; Difficulty concentrating; Feeling hopeless or helpless; Repeated thoughts of death or suicide; Low self-esteem is common with depression. It is also common to have sudden bursts of anger and a lack of pleasure from activities that normally make you happy, including sex.