The scientific definition of a gram is one thousandth of a kilogram. This is a unit of mass, defined as follows:

* Kilogram (kg): The kilogram is the base unit of mass in the International System of Units (SI). It is defined as the mass of the International Prototype Kilogram (IPK), a cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM) in France.

* Gram (g): One gram is equal to 1/1000 of a kilogram.

Therefore, the scientific definition of a gram is based on the kilogram, which is the fundamental standard of mass in the SI system.