The kilogram (kg) is the base unit of mass in the International System of Units.
Definition of the kilogram
It is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 x 10–34 when expressed in the unit J s, which is equal to kg m2 s–1, where the metre and the second are defined in terms of c and ∆νCs.
Definition from the BIPM: Unit of mass
The first definition of the kilogram was "the mass of a liter of water". But this proved impractical and in 1889, the International Prototype Kilogram, a cylinder of platinum-iridium alloy, was made to serve as the reference.
In 1949 the mass of the IPK was found to be deverging from its copies. So, in order to ensure the stability of the unit, a new definition was devised based on the Planck constant and on 20 May 2019 this new definition came into effect.
One kilogram is equivalent to:
With an average of 1.4 kg (3 lbs), ostrich eggs are huge, yet, they are the smallest eggs relative to the size of the adult bird.
The prefix kilo- comes from the ancient Greek chilioi meaning thousand. The associated multiplicative factor is 1000 or 103.