It is referred to as a security principal in Java and Microsoft literature.
[1] Principals can be individual people, computers, services, computational entities such as processes and threads, or any group of such things.
[1] They need to be identified and authenticated before they can be assigned rights and privileges over resources in the network.
A principal typically has an associated identifier (such as a security identifier) that allows it to be referenced for identification or assignment of properties and permissions.
[2] This computer science article is a stub.