Relative density (formerly specific gravity) is a measure of the density of a material. It is dimensionless, equal to the density of the material divided by the density of water. Also, it is measured in no specific unit.
Relative density is often used by geologists and mineralogists to help determine the mineral content of a rock or other sample. Gemmologists use it as an aid in the identification of gemstones. The reason that relative density is measured in terms of the density of water is because that that is the easiest way to measure it in the field. Basically, density is defined as the mass of a sample divided by its volume. With an irregularly shaped rock, the volume can be very difficult to accurately measure. The most accurate way is to put it in a water-filled graduated cylinder and see how much water it displaced. Even this method can be rather inaccurate, though, since it is easy to accidentally spill some water. It is far easier to simply suspend the sample from a spring scale and weigh it under water. Solving Isaac Newton's equations yields the following formula for measuring specific gravity: