The Kilogram:

The base SI unit of mass is the kilogram, represented by the lowercase non-italicized pair of English letters kg. Initially, the kilogram was defined as the mass of 0.001 cubic meter (or 1 litre) of clean liquid water as shown in figure below.

Figure: Initially, the kilogram was defined as the mass of 0.001 cubic meter of clean liquid water.

This is still an outstanding definition, but nowadays scientists have come up with something more absolute. A kilogram is a mass of a sample of platinum iridium alloy which is kept under lock and key at the International Bureau of Weights and Measures.

It is significant to realize that mass is not the similar thing as weight. A mass of 1 kg maintains similar mass no matter where it is placed. That standard platinum-iridium ingot would mass 1 kg on the Moon, on intergalactic space, or in Mars. Weight, in contrary, is a force applied by gravitation or acceleration on a given mass. On the surface of Earth, a 1 kg mass occurs to weigh about 2.2 pounds. In interplanetary space, similar mass weighs 0 pounds; that means it is weightless.