The kilogram was originally defined as the mass of one litre of pure water at standard atmospheric pressure and at the temperature at which water has its maximum density (3.98 degrees Celsius). This definition was hard to realize accurately, partially because the density of water depends slightly on the pressure, and pressure units include mass as a factor, introducing a circular dependency in the definition.
To avoid these problems, the kilogram was redefined as precisely the mass of a particular standard mass created to approximate the original definition. Since 1889, the SI system defines the unit to be equal to the mass of the international prototype of the kilogram, which is made from an alloy of platinum and iridium of 39 mm height and diameter, and is kept at the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). Official copies of the prototype kilogram are made available as national prototypes, which are compared to the Paris prototype ("Le Grand Kilo") roughly every 10 years. The international prototype kilogram was made in the 1880s.