Define relative density of a substance.

(N/A) The relative density of a substance is defined as the ratio of the density of the substance to the density of water at $4^{\circ}C$.
Mathematically,it is expressed as:
$\text{Relative Density} = \frac{\text{Density of substance}}{\text{Density of water at } 4^{\circ}C}$.
Since it is a ratio of two similar physical quantities (densities),it is a dimensionless quantity and has no units.