The meter was again redefined in 1983, but this time in terms of the speed of light. The current definition of the meter was reworded in 2019: “The metre, symbol m, is the SI unit of length. The meter was again redefined in 1983, but this time in terms of the speed of light. Such a change — appropriately made on World Metrology Day — would have ripple effects around the world with regard to how scientific measurements are determined. And what can unit redefinitions teach us about the dynamic history of scientific measurements?Let’s dive into the lengthy history of the meter.The history of standardized length measurements stretches back thousands of years.