The gram (alternative spelling: gramme; SI unit symbol: g) 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 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 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.

The only unit symbol for gram that is recognised by the (SI) is "g" following the numeric value with a space, as in "45 g" to stand for "45 grams" in the English language. The SI does not support the use of abbreviations such as "gr" (which is the symbol for grains), "gm" ("g⋅m" is the SI symbol for gram-metre) or "Gm" (the SI symbol for gigameter).

The gram (SI unit symbol: g) 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 meter, 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 gram is today the most widely used unit of measurement for non-liquid ingredients in cooking and grocery shopping worldwide. Most standards and legal requirements for nutrition labels on food products require relative contents to be stated per 100 g (3.5274 ounces) of the product, such that the resulting figure can also be read as a percentage by weight.