For 130 years, the kilogram was defined as the mass of a polished platinum-iridium cylinder stored in a vault in Paris. By the 1990s, this was the only SI unit still defined by a manufactured object rather than by natural phenomena. That changed in 2019 with a new definition of the kilogram, although one that is rather hard to understand:
One kilogram is defined by fixing the value of the Planck constant—a quantity that appears in quantum physics—to be . The abbreviation for kilogram is kg.
This obscure definition is implemented using a device called a Kibble balance in which an electromagnet is used to balance the weight of a test mass, and the required electric current is measured using quantum standards that depend on the Planck constant. Despite the prefix kilo, it is the kilogram, not the gram, that is the SI unit.