| quantity that has the same value under all circumstances
NOTE 1 – Unless a special name exists, the name of a fundamental physical constant explicitly includes the word "constant". Examples: speed of light c0, elementary charge e, Avogadro constant NA, Faraday constant F = eN0.
NOTE 2 – Recommended values of the fundamental physical constants are published by CODATA.
NOTE 3 – Fundamental physical constants are sometimes used as reference quantities to express the values of other quantities of the same kind as if they were units of measurement. For example, in the field of relativity, it is common practice to express all velocities in terms of the speed of light c0.