9f7dbe57__units___UK_Metric_Association_SI_base_units_it_measures_and_its_definition
[Unit name] kilogram [Symbol] kg
Mass In ordinary language, the kilogram measures what most regard as weight. Strictly speaking, mass is the amount of matter in an object, whereas weight is to do with the pull of gravity (how heavy it is). It was originally just 1000 grams where 1 gram was the mass of 1 cubic centimetre of water (making the kilogram equal to the mass of 1 litre of water). Nowadays it is defined in terms of a much more stable reference in the form of a piece of platinum iridium known as the international prototype. For many years, scientists have been trying to find a better way of defining and measuring it in terms of natural constants in a manner similar to that of the metre and second. A unit of mass equal to the mass of the international prototype of the kilogram.
Base units | UK Metric Association
What it measures and its definition