Kilogram (kg)

The kilogram or kilogramme (kg) is the base unit of mass in the International System of Units (SI) which is the modern standard governing the metric system. The kilogram is defined as being equal to the mass of the International Prototype Kilogramn (IPK), which is almost exactly equal to the mass of one liter of water. The avoirdupois (or international) pound, used in both the Imperial system and U.S. customary units, is defined as exactly 0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds.

In everyday usage, the mass of an object given in kilograms is often referred to as its weight, which is the measure of the gravitational force - or heaviness - of an object. Weight given in kilograms is technically the non‑SI unit of measure known as the kilogram-force. The equivalent unit of force in the avoirdupois system of measurement is the pound-force. In strict scientific contexts, forces are typically measured with the SI unit newton.

The kilogram is the only SI base unit with an SI prefix as part of its name. It is also the only SI unit that is still defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. 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. No final decision is expected before 2015.

Find more here: http://en.wikipedia.org/wiki/Kilogram