The kilogram or kilogramme (SI unit symbol: kg), is the base unit of mass in the International System of Units (SI) (the Metric system) and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK).
The gram, 1/1000th of a kilogram, was originally defined in 1795 as the mass of one cubic centimeter of water at the melting point of water. The original prototype kilogram, manufactured in 1799 and from which the IPK is derived, had a mass equal to the mass of 1.000025 liters of water at 4 °C.
The kilogram is the only SI base unit with an SI prefix (“kilo”, symbol “k”) as part of its name.
More in Wikipedia