The kilogram or kilogrammeThe spelling kilogram is used by the International Committee for Weights and Measures (CIPM) and the U.S. National Institute of Standards and Technology (NIST), but sometimes the spelling kilogramme is also used in British English. (symbol: kg) is the base unit of mass in the International System of Units (SI, from the French lang: Le Système International d'Unités). The kilogram is defined as being equal to the mass of the International Prototype Kilogram

In everyday usage, the mass of an object, which is measured in kilograms, is often referred to as its weight. However, the term weight in strict scientific contexts refers to the gravitational force of an object. Throughout most of the world, force is measured with the SI unit newton and the non-SI unit kilogram-force. Similarly, the [[avoirdupois (or international) pound, used in both the Imperial system and U.S. customary units, is a unit of mass and its related unit of force is the pound-force. The avoirdupois pound is defined as exactly val: u=kg,

 making one kilogram approximately equal to 2.2046 avoirdupois pounds.

Many units in the SI system are defined relative to the kilogram so its stability is important. After the International Prototype Kilogram had been found to vary in mass over time, the International Committee for Weights and Measures (known also by its French-language initials CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. 94th Meeting of the International Committee for Weights and Measures (2005) Recommendation 1: Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants No final decision is expected before 2011.

The nature of mass

While the weight of matter is entirely dependent upon the strength of gravity, the mass of matter is constant (assuming matter is not traveling at a relativistic speed with respect to an observer).According to Einstein's theory of special relativity, the relativistic mass (apparent mass with respect to an observer) of an object or particle with rest mass m₀ increases with its speed as M = γm₀ (where γ is the Lorentz factor). This effect is vanishingly small at everyday speeds, which are by orders of magnitude less than the speed of light, but becomes noticeable at very high speeds. For example, traveling at just 10% the speed of light with respect to an observer—exceedingly fast compared to everyday speeds (about 108 million km/h or 67 million mph)—increases an object's relativistic mass just over 0.5%.

As regards the kilogram, relativity's effect upon the constancy of matter's mass is simply an interesting scientific phenomenon that has zero effect on the definition of the kilogram and its practical realizations. Accordingly, for astronauts in microgravity, no effort is required to hold objects off the cabin floor; they are “weightless.” However, since objects in microgravity still retain their mass and inertia, an astronaut must exert ten times as much force to accelerate a 10 kilogram object at the same rate as a 1 kilogram object.