Final answer:
The term that describes complete mediation in the design of a secure OS is 'Every action follows a policy', ensuring consistent authority checks for each access attempt.
Step-by-step explanation:
In designing a secure operating system (OS), the term that describes complete mediation is 'Every action follows a policy'. This principle ensures that every access to every object must be checked for authority. It is one of the fundamental security principles in system design, which states that a system should validate that an operation or access is allowed for a particular subject or user every time the access is attempted, preventing lapses in security through a single point of control that can be enforced consistently and effectively.
In designing a secure OS, the term that describes complete mediation is 'Every action follows a policy'. Complete mediation means that every action taken by a user or process is checked against a predefined security policy before it is allowed to execute. This ensures that all actions are authorized and aligned with the established security requirements of the operating system.