The kilogram is the last unit of measurement to be untethered from an artificial object.
The watt balance will tell us what a kilogram is.
The General Conference of Weights and Measures comes closer to finding their new standard.
The prototype kilogram we measure all mass against will soon be replaced by more constant standards.
Of course, the whole anthropocentric measurement thing is rather imprecise (Can you imagine designing a microprocessor with cubits?) so, over time, we've come to define units of measurement with various mathematical constants. A meter, for example is...