The wiki page for depression is currently vague and offers little distinction from sad. I propose it be reworked to refer specifically to depression as a mental illness; I believe this is ideal because it provides a clear dividing line between depression and sad.
Here is a proposed draft for the wiki description:
Depression is a mental illness associated with symptoms of prolonged sadness, an inability to feel positive emotions, and general lack of feeling. Victims of depression often feel a desire to self-harm and even to commit suicide. For more information, see articles on WebMD and Wikipedia.
Depression is not to be confused with the simple emotion of sadness; it is characterized by a prolonged feeling of sadness that overwhelms all other aspects of life.