1. Density, mass of a unit volume of a material substance. The formula for density is d = M/V, where d is density, M is mass, and V is volume. Density is commonly expressed in units of grams per cubic centimetre. (Britannica). 2. Mass of a sample or body divided by its volume. (IUPAC). 3. The density of an object is the mass of the object compared to its volume. The equation for density is: Density = mass/volume or D = m/v. Each substance has its own characteristic density because of the size, mass, and arrangement of its atoms or molecules. (American Chemical Society). 4. Density (volumetric mass density or specific mass) is a substance's mass per unit of volume. (Wikipedia)