An accepted reference sample which is used for establishing a unit for the measurement of physical quantities. A physical quantity is specified by a numerical factor and a unit; for example, a mass might be expressed as 8 g, a length as 6 cm, and a time interval as 2 min. Here the gram is a mass unit defined in terms of the international kilogram, which serves as the primary standard of mass. The centimeter is defined in terms of the international meter, which is the primary standard of length and is defined as the length of path traveled by light in a vacuum during a time interval of 1/299 792 458 of a second. In similar fashion, the minute is a time interval defined as 60 s, where the second is the international standard 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 energy levels of the ground state of the cesium-133 atom.