What is the mass per unit volume of a substance called?

Hint: A substance may have different mass or may occupy different volumes but the mass per unit volume can be the same for them. This quantity is something which doesn’t vary with the scale of reference.

Detailed step by step solution:
Matter is defined as something which has mass and occupies a certain volume. Mass of a body may or may not be uniformly distributed throughout the volume of the body. In order to understand the distribution of mass in a body we define a quantity called density.
Density is defined as the mass per unit volume of the substance.
The symbol for density is $\rho $ and the formula to calculate density from mass and volume is
$\rho = \dfrac{M}{V}$
This formula implies that greater the mass of a body, greater is its density while greater the volume occupied by a body, smaller is the density of that body.
The S.I. unit of density is $kg/{m^3}$. Its dimensional formula is $\left[ {{M^1}{L^{ - 3}}{T^0}} \right]$.

Additional information:
Intensive properties are those whose value does not depend on the size of the system. For example, density, pressure, etc.
Extensive properties are those whose value depends on the size of the system. For example, mass, volume, etc.
Basically, the ratio of two extensive quantities gives us an intensive quantity. For example, the ratio of mass and volume which are extensive quantities gives an intensive quantity, the density.

Note: This definition is not limited to mass distribution. We can also define charge density and current density which are used in electrodynamics. Other densities that can be defined are area density or surface density, linear density, relative density or specific gravity, energy density, particle density and so on.