There are seven fundamental units: the metre, kilogram, second, ampere, kelvin, candela, and mole; and two supplementary units: the radian and the steradian. All other units are derived by multiplication or division of these units without the use of numerical factors. SI units in Medicine Expand. International System of Units n. Abbr. SI. A complete, coherent system of units used for scientific work, in which the fundamental quantities are length, time, electric current, temperature, mass, luminous intensity, and amount of substance. Also called International System. SI unit. Any of the units of measure in the International System.