Formalized way of imagining people's thoughts and actions when they use an interface for the first time.
First select a task that the design is intended to support.
Then try to tell a believable story about each action a user has to take to do the task.
To make the story believable, you have to motivate each of the user's actions, relying on the user's general knowledge and on the prompts and feedback provided by the interface. If you can't tell a believable story about an action, then you've located a problem with the interface.