It's the navigational space that your content lives in. Take a museum that's got pictures in it. Rip the pictures out. You have the walls and you have the space that people are moving through. You can choose that.
Interaction design is what people do with their bodies.
Notice how that affects the way they perceive your content. If we look at this another way, we've got the person, the content and all these things going on in between. The person acts through the interface, navigating the content, which is organized by the interface and attended to by the person. That's where our work needs to be. It's not the content.