Final answer:
The kilogram (kg) is the SI unit for mass, previously defined by a platinum-iridium cylinder's mass but now based on fundamental physical constants using a Kibble balance for precise measurements.
Step-by-step explanation:
The SI unit for measuring mass is the kilogram (abbreviated as kg). Originally, the kilogram was defined as the mass of a platinum-iridium cylinder, which was kept at the International Bureau of Weights and Measures near Paris, with exact replicas distributed globally, including at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland.
However, in May 2019, the definition of a kilogram was updated to be based on fixed fundamental physical constants rather than a physical object, to avoid the problems of the object's mass changing over time. This contemporary definition utilizes the Planck constant and is operationalized through the use of a Kibble balance, which allows for a highly precise measurement of mass based on electrical currents that correspond to Planck's constant.