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What do we mean by density, relative density & specific gravity?

Density is mass per unit volume.

Relative Density is a dimensionless ratio of the densities of two materials. Relative density is dimensionless, since it is a ratio between two quantities of same unit. If the ratio is greater than 1, the object will be heavier than the same volume of the reference. If it is less than 1, it will be lighter than the reference. It is important to specify the reference material when reporting a relative density, but when the reference material is not specified it is usually understood to be water.

The term Specific Gravity is similar to Relative Density, except that the reference material is water.

Thus, Specific Gravity is a special case of relative density. While specific gravity has a reference density of water, relative density can have any reference density that is used.