Don Norman's Three Conceptual Models

In human-computer interaction, three distinct models coexist during every interaction:

1. The Design Model: The designer's internal mental conception of how the product ought to function.
2. The System Model (or System Image): The physical interface, code, documentation, signals, and affordances that actually manifest on screen.
3. The User's Mental Model: The user's internalized mental simulation of how the system works, formed by past experience, digital conventions, and observed interface cues.