Système International d'Unités
(SI - International System of Units) The standard set of units of measurement set by the 11th General Conference on Weights and Measures in 1960. There are seven base units: the metre (length), the kilogram (mass), the second (time), the ampere (electrical current), the kelvin (temperature), the mole (number of atoms) and the candela (luminous intensity). These are defined either in terms of physical properties such as the speed of light or, in the case of mass, by a "prototype" lump of platinum-iridium kept at BIPM. Derived units like meters per second (speed) are formed by combining base units. SI also specifies a list of prefixes (multipliers like "k" for 1000).