Colloquially, we often use the terms sadness and depression interchangeably. People will say, for example, that they feel so depressed that summer vacation is ending or they feel so depressed that their car broke down. In medicine, when we use the term depression, we're referring to Major Depressive Disorder - a specific condition that is very different than regular sadness. Sadness is part of the human spectrum of normal emotion. People feel sad at certain times and it's appropriate to feel sad and certain times. Depression, on the other hand, is a mental illness that warrants treatment in many situations.