Density is a quality that measures mass per unit volume. Mathematically, density is found by dividing an object's mass by its volume. Another way to think of it is as relative heaviness. The Greek letter rho and the Latin letter D are common scientific abbreviations used to represent density.
To understand density it is also important to understand the definitions of volume and mass. Volume is the space an object occupies, and mass is the amount of matter it contains.
A substance such as iron has a higher density than a substance such as water. This means that when equal masses of iron and water are compared, the iron has less volume and takes up less space than the water.