A corporation sole is a legal entity with only one member. The classic example is bishops. There is a legal person which is the bishop of, say, Winchester, regardless of which human being, if any, currently occupies that role.
Historically, this was one of the earliest notions of being a legal entity distinct from one or more human beings: if you had made a contract with the bishop and he died, neither you nor his successor could walk away from the contract just because a new individual was now the bishop.