section_path: Definition/SI defining constants
section_name: SI defining constants
paragraph_text: SI defining constants
  
  The seven defining constants are the most fundamental feature of the definition of the system of units. The magnitudes of all SI units are defined by declaring that seven constants have certain exact numerical values when expressed in terms of their SI units. These defining constants are the speed of light in vacuum c, the hyperfine transition frequency of caesium Δ ν, the Planck constant h, the elementary charge e, the Boltzmann constant k, the Avogadro constant N, and the luminous efficacy K. The nature of the defining constants ranges from fundamental constants of nature such as c to the purely technical constant K. The values assigned to these constants were fixed to ensure continuity with previous definitions of the base units.