kilogram -
Hence, the unit of weight such that one-kilogram mass is also a one-kilogram weight.
Wikipedia
The kilogram is the base unit of mass in the International System of Units (SI, from the French Le Système International d’Unités), which is the modern standard governing the metric system. The kilogram is defined as being equal to the mass of the International Prototype Kilogram (IPK), which is almost exactly equal to the mass of one liter of water. It is the only SI base unit with an SI prefix as part of its name.