What universal physical reference now defines the meter in the modern International System of Units (SI)?

The distance light travels in a vacuum during a specific fraction of a second.

The evolution of the metric system into the International System of Units (SI) involved a crucial philosophical shift: moving away from physical artifacts susceptible to wear, damage, or degradation, towards definitions based on universal, unchangeable physical constants. For the meter, this meant abandoning the reliance on the initial terrestrial measurement or the subsequent platinum artifacts. The modern definition anchors the meter to the speed of light, specifically defining it as the distance that light covers when propagating through a vacuum over an extremely small, precisely defined fraction of a second. This reference point is considered infinitely more stable and universally accessible than any physical object, ensuring absolute congruence in measurements across the globe.