Define relative density give its mathematical formula

Define density give its SI unit

The relative density of a substance is defined as the ratio of the density of that substance to the density of water at 4^{o}C.

It is also defined as the ratio of the mass of substance to the mass of equal volume of water at 4^{o}C.

i.e., R.D. = Mass of the substance / mass of an equal volume of water at 4^{o}C.