The base unit of mass in the metric system is the kilogram. Represented by the symbol 'kg', it is defined as the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder kept at the International Bureau of Weights and Measures. The kilogram serves as a fundamental unit in various scientific and engineering applications, providing a standard measure for expressing mass. This unit is crucial for calculations in physics, chemistry, and everyday measurements, ensuring consistency and accuracy in data reporting.