Least privilege is the idea of giving a subject or process only the privileges it needs to complete a task.  Compartmentalization is a technique to separate code into parts on which least privilege can be applied, so that if one part is compromised, the attacker does not gain full access.  Why does this get confused all the time with separation of privilege?  Separation of privilege is breaking up a *single* privilege amongst multiple, independent components or people, so that multiple agreement or collusion is necessary to perform an action (e.g., dual signature checks).  So, if an authentication system has various biometric components, a component that evaluates a token, and another component that evaluates some knowledge or capability, and all have to agree for authentication to occur, then that is separation of privilege.  It is essentially an “AND” logical operation;  in its simplest form, a system would check several conditions before granting approval for an operation.  Bishop uses the example of “su” or “sudo”;  a user (or attacker of a compromised process) needs to know the appropriate password, and the user needs to be in a special group.  A related, but not identical concept, is that of majority voting systems.  Redundant systems have to agree, hopefully outvoting a defective system.  If there was no voting, i.e., if all of the systems always had to agree, it would be separation of privilege.  OpenSSH’s UsePrivilegeSeparation option is *not* an implementation of privilege separation by that definition, it simply runs compartmentalized code using least privilege on each compartment.