The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), the metric system, 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 metre, both based on fixed fundamental constants of nature. This allows a properly-equipped metrology laboratory to calibrate a mass measurement instrument such as a Kibble balance as the primary standard to determine an exact kilogram mass.
The kilogram was originally defined in 1795 as the mass of one litre of water. The current definition of a kilogram agrees with this original definition to within 30 parts per million.
In 1799, the platinum Kilogramme des Archives replaced it as the standard of mass. In 1889, a cylinder of platinum-iridium, the International Prototype of the Kilogram (IPK), became the standard of the unit of mass for the metric system and remained so until 2019. The kilogram was the last of the SI units to be defined by a physical artefact, although precision kilogram masses remain in use as secondary standards for ordinary purposes.