What Is a Dimension?

The universe started out like ten dimensional, and there were these beings that were fighting each other. They would collapse the universe from ten dimensins down to nine before their enemies could adapt. The idea in this book is that they have some weapon which can collapse space from a certain number of dimensions to one fewer. So you're smashing anything that used to be in three d your smashing it now down to two dwhich makes it pretty hard to survive.