Using multiple access controls in a a series. So when one fails, it is not over for you.
Means simplifying the classifying of objects or assigning of roles to subjects.
For example: Job functions (e.g., manager, employee, contractor) or classify documents by their sensitivity level (e.g., confidential, secret, top-secret)
Means not showing data to unauthorized subjects. The data should not even be discoverable.
Is not a form of protection, but more that discovering things is more difficult by doing something unconventional, like using an old, obscure communication protocol.
Use good encryption to hide data or communication from unauthorized users. Weak or poor encryption is nothing more than security through obscurity