Meter
Standardized: 1983
Measures: Length
Definition: The distance traveled by light in a vacuum in 1/299,792,458 second
Second
Standardized: 1967
Measures: Time
Definition: The time it takes for a cesium-133 atom to cycle 9,192,631,770 times between two specific quantum states
Ampere
Standardized: 1948
Measures: Electrical current
Definition: The current required to create a force of 2 x 10-7 newtons per meter between two parallel wires
Kelvin
Standardized: 1954
Measures: Temperature
Definition: 1/273.16 the temperature of the triple point of water — when it’s simultaneously gas, liquid, and solid
Mole
Standardized: 1971
Measures: Amount of stuff
Definition: The number of atoms in 12 grams of carbon 12 (6.022 x 1023)
Candela
Standardized: 1979
Measures: Brightness
Definition: The intensity of a 1/683-watt yellow-green light spread over a square meter, seen from a meter away
of course you realize these are all quite aribitrary. i could start a unit like the ‘prick’ unit and give you a score of 1.3 and you wouldn’t know what the means , , ,