We compare the density of different substances with the density of water at 4°C.
Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance".
4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only.
There is no unit of relative density as it is the ratio of two similar quantities.
The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7.
It means that the substance is 7.7 times heavier than water.