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Death is a ranking title. It is the designation of whoever delegates the position in place of Death himself. Sometimes, the title Death is used to identify Reapers, as they are heavily involved in the natural order and the afterlife.
Death held this title for milenia. Being Death itself as both a primordial entity and the reapers' superior, this position afforded him both the respect and fear from everything that existed, even God himself. It was from this fear, that Death was bound and buried in a magical casket six hundred feet under the earth's surface.
There are times when the title of Death is granted to someone else. When Dean bargained with Death over Sam's soul, a wager was struck that if Dean could do Death's job successfully for one day, than he would help Dean get Sam's soul back. During the bet while wearing the ring, Dean took to calling himself "Death", and became addressed and viewed as such during the "bet".
Years later, Dean killed Death with his scythe. Unbeknownst to Dean, when an incarnation of Death dies, the next reaper to die will take their place; indeed, when the reaper Billie was killed by Castiel, she became the new Death. To her it was like "getting a promotion", as she had rose to a higher understanding of the natural order, as well as gaining her own scythe and ring.