According to specialists in psychology and psychiatry, depression includes these symptoms: sad moods, self-image distortions, sleep problems, hopeless feelings, changes in appetite, fatigue, among others.
If you struggle with depression, I encourage you to have a medical check up first. When medical causation (e.g. brain chemistry, organ failure etc.) is in play, then no amount of talk therapy or support will resolve the physiological component.
At its root, depression is a psychological, emotional, and spiritual disease. After the medical component is ruled out or taken cared of, then you can begin to address the very heart or deepest essence of depression which is essentially non-physical.