New metrics for the metric system

The Systeme Internationale before the redefinition: Dependence of base unit definitions on other base units (for example, the metre is defined in terms of the distance travelled by light in a specific fraction of a  second), with the constants of nature and artefacts used to define them (such as the mass of the International Prototype Kilogram for the kilogram).

The SI system after the redefinition: Dependence of base unit definitions on physical constants with fixed numerical values and on other base units that are derived from the same set of constants.

The new definitions will come into effect next year.