2 Answers

Stress and Depression
This is my personal opinion.
Depression is a feeling of remorse, sadness, hopelessness, despair (are you getting the picture) brought about by circumstances in one's life (job, family, health, social) that are less than ideal OR by a chemical imbalance in the brain.  Depression can be temporary (postpartum depression or relief from a life circumstance), or it can be permanent (chemical imbalance). Depression is often treated with medication.
Stress is the life circumstance STUFF that causes depression.  A  feeling of need and/or pressure upon oneself to be proactive or defensive about a situation is stress. . 

STRESS, is a response to any event which is perceived to alter or threaten our well–being.

Depression is pretty much a "Head" thing, IMO

Depression is the "IN THING" that the shark Lawyers are now using to get people full "Social Security" disability and Medicare benifits.    S.S. disability payment  is equal to what a person at 65 would collect.

They fight for you with S. S. Officials then take 1/2 of you back dated benifits and half of your first six months benifits, as the fee.

Depression is akin to the old hard to prove, "back injury," to get something for nothing.