Define density and provide its $SI$ unit.

(N/A) Density of a substance is defined as its mass per unit volume.
The formula for density is: $D = \frac{m}{V}$
$SI$ units of density are derived as follows:
$SI$ unit of density $= \frac{\text{SI unit of mass}}{\text{SI unit of volume}} = \frac{kg}{m^{3}} = kg \ m^{-3}$
Since this unit is quite large,chemists often express density in $g \ cm^{-3}$,where mass is measured in grams and volume in $cm^{3}$.