Definition of depression: Sadness, feeling down, and having a loss of interest or pleasure in daily activities are familiar feelings for all of us. But if they persist and affect our lives substantially, the issue may be depression. To be easy,  depression is a mood disorder that causes a persistent feeling of sadness and loss of interest. Also called major depressive disorder or clinical depression, it affects how you feel, think and behave and can lead to a variety of emotional and physical problems. You may have trouble doing normal day-to-day activities, and sometimes you may feel as if life isn’t worth living.