SI base unit
Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre [1 cm3], and at the temperature of melting ice" (later at 4 °C, the temperature of maximum density of water). However, in a reversal of reference and defined units, a gram is now defined as one thousandth of the SI base unit, the kilogram, or 1×10−3 kg, which itself is defined by the International Bureau of Weights and Measures, not in terms of grams, but by taking the fixed numerical value of the Planck constant to be kg⋅m2⋅s−1.