gram (g), unit of mass or weight that is used especially in the centimetre-gram-second system of measurement (see International System of Units). The gram is very nearly equal (it was originally intended to be equal; see metric system) to the mass of one cubic centimetre of pure water at 4 °C (39.2 °F), the temperature at which water reaches its maximum density under normal terrestrial pressures. The gram is now defined as 0.001 kilogram (kg), which is defined in terms of Planck’s constant. The gram of force is equal to the weight of a gram of mass under standard gravity. For greater precision, the mass may be weighed at a point at which the acceleration due to gravity is 980.655 cm/sec2. The official International System of Units abbreviation is g, but gm has also been used.