Relative density (RD) of a substance is defined as the ratio of the density of the substance to density of water at 4oC.
R.D 
This RD of a substance is also defined as the ratio of the mass of the substance to mass of equal volume of water.
R.D has no unit as it is a mere ratio.
In C.g.s system density of water at 4oC is 1 g cm-3
= numerical value of density of the substance.
For example density of mercury is 13.6 g cm-3, it R.D is 13.6
In S.I system density of water is 1000 kg m-3 and that of mercury is 13.6 x 103 kg m-3.
R.D of mercury = 
R.D of a substance has the same value in any system.
R.D of a substance indicates how much heavier a certain volume of the substance is as compared with water of same volume.
