Depression can be a very serious medical issue. Referring to depression as a mood is describing a feeling, a sense that all is not right with the person affected, a world which for a length of time feels very dark to the depressed person. Depending on how long that feeling lasts is a good indicator of how severe the illness is. A symptom is something that describes one or more issues a person is having which indicate depression. For example, a symptom for one person may be that appetite may be significantly decreased or the desire to sleep may be significantly increased to abnormal levels such as sleeping 22 hours of every 24. A syndrome is a broader spectrum of depression usually when another condition accompanies the depression; for example, anxiety or phobias can be part of one person's depression but not another person. As I have a sister who deals with clinical depression to the point of a psychic break with the real world, depression must be treated by a physician who truly knows how to handle severe depression. Her doctor is a psychiatrist. Many people talk to a physician without checking out their credentials for handling depression. Severe depression can be life threatening as my son's best friend began taking anti-depressants for severe depression and ended up a suicide when they didn't help immediately.