is defined as the closeness of materials.  You are probably already familiar with density.  A dense crowd is one in which many people are packed close together.  Can you think of other examples of density from your daily life?

Mathematically, density is defined as follows:

Let's calculate the density of each rectangle in the picture below:

Each rectangle has the same volume.  Let's assume that this volume is 20 mL.  Each brown spot weighs 1 gram.  So the density of the upper rectangle is the sum of the weights of the brown spots divided by the volume:

And the density of the lower rectangle is:

The lower rectangle has a much lower density because there are fewer brown spots in the same unit of volume.