Gram
The gram (alternative British English spelling: gramme;SI unit symbol: g) (Greek/Latin root grámma) is a metric system unit of mass. Gram can be abbreviated as gm or g.
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), a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or 1×10−3 kg, which itself is defined as being equal to the mass of a physical prototype preserved by the International Bureau of Weights and Measures.
Official SI symbol
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". The SI does not support the use of abbreviations such as "gr" (which is the symbol for grains), "gm" or "Gm" (the SI symbol for gigametre).
History
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.