Right now a kilogram is defined by a golf-ball sized cylinder locked in a vault in Sèvres, France. The International Prototype of the Kilogram, made of platinum and iridium, otherwise known as Le Grand K or Big K, is the last of its kind. The hunk of metal is only taken out once every 40 years to be compared against similar replicas from around the world.
All other standardized units of measurement have been redefined in terms of a fundamental natural constant. For instance, the meter, which was originally represented by a metal bar, was redefined in 1983 as the distance light travels in a vacuum during 1/299,792,458 of a second.
So for years, physicists have chased an elusive dream: replacing the physical kilogram with a standard inherent in properties of nature such as the speed of light, the wavelength of photons and the Planck constant (also called h-bar), which links the energy a wave carries with its frequency of oscillation. Scientists could use the Planck constant to compare the energy of a wave with Einstein's iconic E=mc2 equation; in that way, they would determine mass solely through the physical constants.
Unfortunately, no one has yet been able to measure the Planck constant to a level of precision that could rival what has been achieved by using Le Grand K as the benchmark. But researchers are making strides, and at the current pace, believe they can redefine the kilogram as soon as 2018.
Here is an article describing the new 'natural' definition of the Kilogram
Redefining The Kilogram 
Re: Redefining The Kilogram
