Metric units of mass equal to about 2.205 pounds. One kilogram is approximately equal to the weight of 1000 cubic centimeters of water. The kilogram is the base unit of mass in the International System of Units (SI) (the Metric system); defined as being equal to the mass of the International Prototype of the Kilogram. It is 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. (en)