Despite what the dictionaries and thesauruses say, depression is not a synonym for sadness. It drives me crazy when people use the words interchangeably. As a person that suffers from depression, I know the difference between the two.
Sadness does not last that long. A person that is feeling sad will feel better shortly. They can also snap out of their sadness or be distracted with happy thoughts. A person who is feeling depressed cannot do that. Depression is a mental illness caused by an imbalance of chemicals in the brain. Despite popular belief, depression is more than sadness. Sure, there is overwhelming sadness that a person can do nothing about, but there are also the feelings of worthlessness, hopelessness, and helplessness. A person suffering from depression also has loss of interest or pleasure in ordinary activities, sleep disturbances, and changes in appetite. Depression also means thoughts of suicide and has difficulty concentrating.
Is depression and sadness the same thing? Definitely not. If it was, a person could snap out of there depression with a positive attitude. It also would not last as long.