In particle physics and physical cosmology, Planck units are a set of units of measurement defined exclusively in terms of four universal physical constants, in such a manner that these physical constants take on the numerical value of 1 when expressed in terms of these units. The four universal constants that, by definition, have a numeric value 1 when expressed in these units are: * the speed of light in a vacuum, c, * the gravitational constant, G, * the reduced Planck constant, ħ, * the Boltzmann constant, kB.