Density is defined as mass per unit volume.
The higher the number, the more closely spaced are the people. Thus, we would say the room is more densely packed with people. In relating our room analogy to substances, we can use the point mass described earlier as we did the number of people. Consider a simple geologic example of an ore body buried in soil. The density of the material can be thought of as a number that quantifies the number of point masses needed to represent the material per unit volume of the material just like the number of people per cubic foot in the example given above described how crowded a particular room was.