Gram

Gram
The mass of this pen cap is about 1 gram
Unit system	SI derived unit and CGS base unit
Unit of	Mass
Symbol	g
1 g in ...	... is equal to ...
SI base units	10−3kilograms
CGS units	1 gram
Imperial units U.S. customary	0.0353 ounces

The gram (alternative British English spelling: gramme;SI unit symbol: g) (Greek/Latin root grámma) is a metric system unit of mass.

Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" (later 4 °C, the so-called triple-point of water where ice, liquid, and vapor can undergo phase transformation from any one to any other phase—the reason it was attractive as a standard). However, in a reversal of reference and defined units, a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or 1×10⁻³ kg, which itself is now defined, not in terms of grams, but as being equal to the mass of a physical prototype of a specific alloy kept locked up and preserved by the International Bureau of Weights and Measures. This is in the tradition by which many customary local reference standard 'stones', lengths (objects) and weights were required to periodically undergo comparison with the official nations standard referents, usually with a particular periodicity defined by the countries statuate laws.

The only unit symbol for gram that is recognised by the International System of Units (SI) is "g" following the numeric value with a space, as in "640 g" to stand for "640 grams" in the English language. The SI does not support the use of abbreviations such as "gr" (which is the symbol for grains), "gm" (the SI symbols for gram metre) or "Gm" (the SI symbol for gigametre).

The word gramme was adopted by the French National Convention in its 1795 decree revising the metric system as replacing the gravet introduced in 1793. Its definition remained that of the weight (poids) of a cubic centimetre of water. French gramme was taken from the Late Latin term gramma. This word, ultimately from Greek γράμμα "letter" had adopted a specialised meaning in Late Antiquity of "one twenty-fourth part of an ounce" (two oboli), corresponding to about 1.14 (modern) grams. This use of the term is found in the carmen de ponderibus et mensuris ("poem about weights and measures") composed around 400 AD. There is also evidence that the Greek γράμμα was used in the same sense at around the same time, in the 4th century, and survived in this sense into Medieval Greek, while the Latin term did not remain current in Medieval Latin and was recovered in Renaissance scholarship.