What is the basic unit of mass in the International System of Units?

The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering, and commerce worldwide, and is often simply called a kilo colloquially. It means 'one thousand grams'. The kilogram is defined in terms of the second and the meter, both of which are based on fundamental physical constants. The kilogram was originally defined in 1795 during the French Revolution as the mass of one liter of water. The current definition of a kilogram agrees with this original definition to within 30 parts per million.

Source: Wikipedia