The metre is the basic unit of length. It is the distance light travels, in a
vacuum, in 1/299792458th of a second.
The kilogram is the basic unit of mass. It is the mass of an international
prototype in the form of a platinum-iridium cylinder kept at Sevres in France.
It is now the only basic unit still defined in terms of a material object, and
also the only one with a prefix [kilo] already in place.
The second is the basic unit of time. It is the length of time taken for
9192631770 periods of vibration of the caesium-133 atom to occur.
The ampere is the basic unit of electric current. It is that current which
produces a specified force between two parallel wires which are 1 metre apart in
a vacuum. It is named after the French physicist Andre Ampere (1775-1836).
The kelvin is the basic unit of temperature. It is 1/273.16th of the
thermodynamic temperature of the triple point of water. It is named after the
Scottish mathematician and physicist William Thomson 1st Lord Kelvin
The mole is the basic unit of substance. It is the amount of substance that
contains as many elementary units as there are atoms in 0.012 kg of carbon-12.
The candela is the basic unit of luminous intensity. It is the intensity of a
source of light of a specified frequency, which gives a specified amount of
power in a given direction.