A second is 9,192,631,770 times the period of the radiation emitted when an electron moves between the two lowest energy states in a caesium atom with 78 neutrons kept at a temperature of absolute zero.

The metre is defined as the distance that light in a vacuum travels in one 1/299,792,458 th of a second.

The current definition of the kilogram is "we've got this lead brick in Paris and it's that"

The ampere (i.e. amp) is the amount of electric current that if run through two perfectly straight, perfectly parallel, infinitely long wires of infinitesimal thickness and placed one metre apart in a perfect vacuum would cause the two wires to cause a force of 2×10^−7 N on each other.

N is newtons which is the force needed to accelerate a mass of one kilogram by one metre per second per second.

So. Next time you here you hear someone saying some electric current is five amps that means that it's five times the amount of electric current that if run through two perfectly straight, perfectly parallel, infinitely long wires of infinitesimal thickness and placed the distance that light in a vacuum travels in one 1/299,792,458 th of 9,192,631,770 times the period of the radiation emitted when an electron moves between the two lowest energy states in a caesium atom with 78 neutrons kept at a temperature of absolute zero in a perfect vacuum would cause the two wires to cause a force equivalent to 2×10^−7 times the force required to accellerate this lead brick we got in Paris at a rate of the distance that light in a vacuum travels in one 1/299,792,458 th of 9,192,631,770 times the period of the radiation emitted when an electron moves between the two lowest energy states in a caesium atom with 78 neutrons kept at a temperature of absolute zero per 9,192,631,770 times the period of the radiation emitted when an electron moves between the two lowest energy states in a caesium atom with 78 neutrons kept at a temperature of absolute zero per 9,192,631,770 times the period of the radiation emitted when an electron moves between the two lowest energy states in a caesium atom with 78 neutrons kept at a temperature of absolute zero.