A decedent is someone who has died. Decedents are deceased.
Every language has ways to avoid saying the dead guy, and English has two that come from the same root: deceased, a formal and impersonal way of designating one recently departed, and decedent, the version preferred when a lawyer is in the room. Both words are from Latin decedere, meaning "to die." Decedents include all dead people, no matter when they died. Someone who died an hour ago is a decedent as well as someone who died 300 years ago.