Sometimes I think depression could also be called emotional suppression or recession, or even emotional retrogression. But, to me it's more than just feeling sad, in fact, sadness is not an emotion I feel during depression. It's more like a "dead" feeling with general lack of enthusiasm and will to live/enjoy life, but it's so overwhelming that it begins to have a negative impact on the person's life. It's a state of being, not a feeling.