The 11th General Conference on Weights and Measures redefines the metre in terms of a specific number of wavelengths of light of a specific frequency in a vacuum. It is defined to be 1 650 763.73 wavelengths of the electromagnetic radiation emitted by an electron transitioning between the levels 2p10 and 5d5 of the krypton-86 (86Kr) atom in a vacuum. The metre is now defined in terms of reproducible physical effects, rather than a synthetic artifact.
The 17th General Conference on Weights and Measures redefines the metre in terms of the distance traversed by light (having a universally constant speed in a vacuum) in a specific time. It is defined as the distance traveled by a beam of light in a vacuum in 1/299 792 458second. This is the current definition. This has the effect that the speed of light is defined to be exactly 299 792 458 m/s.