"Density" sometimes means the ratio between other properties instead of mass and volume. Then it means how much of a property can be found in a specific piece of what they are looking at. For example, "population density" is how many people live within the same amount of land area. The population density in the city is higher than the country side because people live closer to each other in the city. In computers, "storage density" is how much data can fit on a data storage device in relation to its physical size. A Blu-ray disc has a higher storage density than a DVD which has a higher storage density than a CD, even though they all have almost exactly the same volume.