The gram or gramme, symbol g, is a unit of mass, and is defined in the SI system of units as one one-thousandth of a kilogram (i.e., 1x10-3 kg). The kilogram, in turn, is defined as the mass of a specific platinum-iridium cylinder maintained at the International Bureau of Weights and Measures in Paris, France.
Although the gram is not an SI base unit, it is a derived unit of the kilogram, which is a base unit. However, the gram is a base unit of the older cgs system of measurement, a system which is no longer widely used. Regardless, the gram is an essential measurement in scientific endeavors worldwide.
A gram was originally defined as the weight of one cubic centimeter of water at its densest. This occurs at a temperature near four degrees Celsius. However, its definition was changed to the mass of a metal artifact in the eighteenth century.
milligram << gram << kilogram