Interactive Density refers to the amount of visual engagement viewable on a user interface; like a web page or software application screen. A graphic user interface that has a large amount of interactive density on a page may be longer for the user to decipher what to do.
This differs from spirit and volume in that the volume of content has a depth of interactivity, the combined volume and depth of the interactivity gives us the density. The depth of the interactivity is categorized by the interactive volleys available in that space without going to another page.
Mike Hasselmann, Business Analyst and Information Architect