Thermodynamic properties differ in how they scale with system size.

An extensive property scales with the amount of matter. If two identical systems are combined, their mass $m$ and volume $V$ double.

An intensive property does not scale with system size. Temperature $T$, pressure $p$ and density $\rho$ remain the same when two identical equilibrium systems are combined.

Dividing an extensive property by mass produces a specific property. The most immediate example is specific volume:

$$v=\frac{V}{m}.$$

Because density is $\rho=m/V$,

$$v=\frac{1}{\rho}.$$

For example, if $2\ \mathrm{kg}$ of a substance occupies $0.50\ \mathrm{m^3}$, then

$$v=\frac{0.50}{2}=0.25\ \mathrm{m^3/kg},\qquad \rho=4\ \mathrm{kg/m^3}.$$

The same construction can be applied to other extensive properties once they are introduced. Specific properties let thermodynamic relations describe systems independently of their total mass.