nLab
kilogram

The kilogram (kilogramme, $\mathrm{kg}$) is a unit of mass. It is $1000$ grams (gramme, $g$).

One kilogram may soon be defined to be the mass of $N$ atoms of the carbon? isotope? ${\mathrm{C}}^{12}$, where $N$ is $\frac{1000}{12}$ times the Avogadro number. Currently, however, this fact is actually the definition of the Avogadro number; instead, the kilogram is defined as the mass of a certain physical object kept by the BIPM in Paris.