What are the seven base SI units?

The seven base SI units are metre, kilogram, second, ampere, kelvin, mole, and candela.

The metre is the unit of length and is defined as the distance travelled by light in a vacuum in 1/299,792,458 of a second. The kilogram is the unit of mass and is defined as the mass of the International Prototype of the Kilogram, which is a platinum-iridium cylinder kept at the International Bureau of Weights and Measures. The second is the unit of time and is defined as 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 caesium-133 atom.

The ampere is the unit of electric current and is defined as the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2 x 10^-7 newton per metre of length. The kelvin is the unit of temperature and is defined as the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

The mole is the unit of amount of substance and is defined as the amount of substance that contains as many elementary entities as there are atoms in 0.012 kilograms of carbon-12. The candela is the unit of luminous intensity and is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.