What is density?
Density is a physical characteristic, and calculates of mass per unit of volume of a material or substance. It is a measurement of the quantity of matter in a given volume of something.
The higher an object's density, the higher it's mass per unit of volume. The average density of an object equivalent its total mass divided by its total volume. A denser object (like iron) will have less volume than an equal mass of some less dense substance (like water).