Sadness;feeling blue,cheerless,downcast, gloomy,glum,mournful etc etc.
You can be sad and any of the above for a variety of reasons. To lose a friend for example whether it be in cyber space or the real world. Sad because you have hurt someone or they have hurt you.Sad because you have lost a loved pet. so many things create sadness but depression is a different ball game. You can get over any of the above but depression can last many years.
My doctor diagnosed me with the medical term neurostrenia. I had lost the ability to cope on a day to day basis due to a simple internal operation that went wrong and resulted in me being re-admitted to hospital several times.I was bleeding internally. I thought it was the big C but it wasnt.
That and losing my parents in during this time made me so depressed that I was advised to change my job and take another direction. In your 50s that was difficult.
Depression; dejection.despair,despondency,hopelessness and low esteem. These are just a few definitions of the two conditions.
Sadly many people think the either is all imagination. They are so wrong in their assessment.
My heart goes out to sufferers of depression. You can feel suicidal at times. But I dont think sadness is on the same level.
I speak from experience and not from a qualified professional position.