The gram (originally gramme; SI unit symbol g) is a unit of mass in the International System of Units (SI) equal to one one thousandth of a kilogram.

Originally defined as of 1795 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", the defining temperature (~0 °C) was later changed to 4 °C, the temperature of maximum density of water.

However, by the late 19th century, there was an effort to make the base unit the kilogram and the gram a derived unit. In 1960, the new International System of Units defined a gram as one one-thousandth of a kilogram (i.e., one gram is 1×10⁻³ kg). The kilogram, as of 2019, is defined by the International Bureau of Weights and Measures from the fixed numerical value of the Planck constant (h), which is 6.62607015×10⁻³⁴ kg⋅m²⋅s⁻¹.