Wikipedia says:

In 1988, Donald Norman appropriated the term affordances in the context of human−machine interaction to refer to just those action possibilities that are readily perceivable by an actor.

This captures well what I remember from the book “The Design of Everyday Things”. In a security context there is another requirement: It must be understood in some sense what actions are impossible by others. There are several critical words that must be clarified in the above—primarily “others”. Also “impossible” may refer to acts carried out with proximity to the computer and acts possible from far away. Norman’s solutions to these problems do not generally apply to security issues.