The kilogram is the base unit of mass in the SI system of units (and is almost exactly equal to the mass of one litre of water).which is almost exactly equal to the mass of one litre of water.
In everyday usage, the mass of an object given in kilograms is often referred to as its weight. In fact the relationship between the mass
and weight
of an object is
The kilogram is the only SI base unit with an SI prefix as part of its name and is also the only SI unit that is still directly defined by an actual historical artifact rather than a fundamental physical property that can be reproduced in different laboratories. Because four of the seven base units in the SI system are defined relative to the kilogram so its stability is important.
The International Prototype Kilogram (shown above) is kept in the custody of the International Bureau for Weights and Measures(BIPM) who hold it on behalf of the General Conference on Weights and Measures (CGPM). After the International Prototype Kilogram had been found to vary in mass over time, the International Committee for Weights and Measures (CIPM) recommended in 2005 that the kilogram be redefined in terms of a fundamental constant of nature. At its 24th meeting the General Conference on Weights and Measures (CGPM) agreed in principle that the kilogram should be redefined in terms of the Planck constant, but deferred a final decision until its next meeting, scheduled for 2014.