means the SI base unit of length, defined by how far light travels in a tiny fraction of a second.

from born in 1793 revolutionary france, meant to be one ten-millionth of the distance from the north pole to the equator; scientists literally trekked from dunkirk to barcelona to survey it, got the math slightly wrong, and the definition has been quietly upgraded ever since — from a metal bar in a paris vault to krypton light waves to, since 1983, the distance light travels in 1/299,792,458 of a second.