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

Frequency equal to one event / second

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.

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

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)

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

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

Definition

Measurement

Distance equal to 0.9144 of a meter

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

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

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

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

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

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