The duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom

Length of the path traveled by light in a vacuum during a time interval of 1 / 299 792 458 of a second

Force needed to accelerate a mass of 1 kg at 1 meter per second per second

Mean distance from the center of the Earth to the center of the Sun (a little less than 150 million km)

Distance a star shifts position by 1 arcsecond during the time Earth moves 1 a.u. perpendicular to the star (less than 12 is a great Kessel run)

The number of electrons surrounding an uncharged atom (also = number of protons in its nucleus).

Originally, the distance a Roman legion could march in 1000 paces. Now equal to 8 furlongs, 80 chains, 320 rods, 1760 yards or 5280 feet.

Definition

Measurement

Amount of a substance equal to the number of atoms of Carbon-12 in 0.012 kilograms

Originally, defined as the mass of 1 cubic centimeter of pure water

Distance equal to 0.9144 of a meter

Heat needed (at a pressure of one atmosphere) to raise the temperature of one gram of water by 1 degree Celsius

Unit of time equal to the interval between two arrivals of the Sun at the Tropic of Capricorn

Frequency equal to one event / second

1/100 of the difference between the freezing and boiling points of water