I'm not entirely sure if you're talking about a physical condition (of which I've never heard) or a psychological condition. Depression is a low mood. Major depression is mental disorder which lasts a minimum of two weeks during which time the person is sad for most of the time or has lost interest or pleasure in activities most of the time. There's then a whole list of other criteria. Depression has been linked to heart disease although they are still doing research to find out exactly how.