Values that are ascribed to physical quantities to allow them to be discussed precisely. There are several systems of units but the one that is most widely used, and used universally in science, is the Système International d’Unités (SI units). These begin with base units for length or displacement: (metre, m), mass (kilogram, kg), time (second, s), current (ampere, A), temperature (kelvin, K), amount of substance (mole, mol), and luminous intensity (candela, cd). These units are defined in absolute terms. For example, 1 metre is the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second, and 1 second is the time that elapses during 9,192,631,770 cycles of the radiation produced by the transition between the two hyperfine levels of the ground state of the atom of cesium-133. A further set of units is derived from the base units, more derived units being added as the need for them arises. The list of derived units includes: area (m2), volume (m3), speed (m s–1), acceleration (m s–2), concentration (mol m3), and luminance (cd m–2).