Today (20th May 2019) it was the official birth of the new definition of the kilogram (kg) of the International System of Units (abbreviated as SI, from the French "Système International").
The kilogram will no longer be defined based on the mass of the International Kilogram Prototype (IPK), a cylinder made from a platinum-iridium alloy stored in a basement vault of the International Bureau of Weights and Measures in Saint-Cloud, France. Now it will be defined by a relation based on the Planck constant. This is a fixed quantity related to the famous equation E = mc2 and quantum theory, specifying the amount of energy carried within a single particle of light (or photon). And that explanation is very over-simplified!
Why was the change needed?
The problem is that the mass of the International Kilogram Prototype (IPK) cylinder, which was forged in London in 1889 and transported to France, was changing over time - despite it being housed under three glass bell jars to protect it! The standard kilogram unit, by its very definition, cannot change and even though the variation in mass of the cylinder was very small, a solution still needed to be found.
By using the Planck constant, the value of the kilogram is now fixed and cannot change.
This is a historic event in the world of Science and Technology. The IPK cylinder is more casually known as "Le Grand K" and the biggest remaining question is what will be in store for its retirement. Now it will not matter so much if it gains some weight as it gets older!