What is the Kilogram ?

Source: en.wikipedia.org

" The kilogram (also kilogramme) is the base unit of mass in the International System of Units (SI), the metric system, having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. It means 'one thousand grams'.
The kilogram was originally defined in 1795 as the mass of one litre of water. (...) "