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, also known as "La Grande K" or "Big K").

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 1.000028 dm³ of water at its maximum density at approximately 4 °C.

The kilogram is the only SI base unit with an SI prefix ("kilo", symbol "k") as part of its name. It is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. Three other base units (Cd, A, mol) and 17 derived units (N, Pa, J, W, C, V, F, Ω, S, Wb, T, H, kat, Gy, Sv, lm, lx) in the SI system are defined relative to the kilogram, so its stability is important. Only 8 other units do not require the kilogram in their definition: temperature (K, °C), time and frequency (s, Hz, Bq), length (m), and angle (rad, sr).