Every measurable quantity in physics needs an agreed unit so that scientists everywhere report the same number for the same amount. For mass, the internationally agreed base unit under SI is the kilogram, written kg. It is worth noting that the kilogram is unusual among SI base units because it already carries a prefix, kilo, in its name, unlike the metre for length or the second for time. Smaller or larger masses are expressed using this same unit with prefixes, such as grams (one thousandth of a kilogram) or tonnes (one thousand kilograms), but the official SI base unit itself remains the kilogram. So the answer is kilogram, symbol kg.