Question

Why reltive density is called specific density

Answer 1

because in relative density the density of any substance is measured in respect of density of water.
relative density = density of any substance/density of water.

Answer 2

Hello your answer ....

As we all know that relative density is the ratio of the density of substance to the density of a given reference material
✒Specific gravity means density with respect to water .
✒And to find the relative density $\bold{water}$ is taken as reference material .
✒So,reltive density is also called specific gravity