Kilogram (kg)
Definition: The kilogram (symbol: kg) is the SI base unit of mass. Since 2019, it is defined by fixing the numerical value of the Planck constant to exactly 6.626 070 15 × 10^-34 joule-seconds, replacing the previous definition based on a physical platinum-iridium cylinder.
History/origin: The kilogram was originally defined in 1795 as the mass of one liter of water at 4 °C. In 1889 it was redefined as the mass of the International Prototype of the Kilogram (IPK), a physical artifact kept in France. The 2019 redefinition tied it to a fundamental physical constant instead.
Current use: The kilogram is the standard unit of mass in virtually every country. It is used in commerce, science, food packaging, and health. Body weight, food portions, and shipping weights are routinely expressed in kilograms worldwide.
