Here is an interesting definition I found:
http://www.worldtrans.org/lyssa/density.html
Density: Density denotes a vibrational frequency and not a location, which the term "dimension" implies. The density structure of this reality is primarily expressed in seven levels, though each level has sub-levels within it. The density scale is a model used to communicate one's perception of orientation in relation to other realities.
Dimension: Dimension refers to one's location in space/time rather then a person's vibrational frequency (density). Webster defines "dimension" as "Magnitude measured in a particular direction, specifically length, breadth, thickness or time." There are an infinite number of dimensions existing with a given density or vibrational frequency.
Namaste, Steven