Density is a measurement of the mass of a substance per unit volume, defined as mass divided by volume. It can be used to compare how compact or dense different materials are. Examples given include lead having the highest density at 11.3 g/cm3, while carbon monoxide is the least dense at 0.00125 g/cm3. The document provides instructions on calculating density by taking the mass of an object and dividing by its volume in cm3, and includes practice problems asking the reader to calculate densities of minerals with given masses and volumes.