In physics, a dimensionless physical constant (sometimes fundamental physical constant) is a universal physical constant that is dimensionless – having no unit attached, so its numerical value is the same under all possible systems of units. The best known example is the fine structure constant α, with the approximate value 1/137.036.
However, the term fundamental physical constant has also been used (as by NIST) to refer to universal but dimensional physical constants such as the speed of light c, vacuum permittivity ε0, Planck's constant h, or the gravitational constant G.
Read more about Dimensionless Physical Constant: Introduction, Constants in The Standard Model and In Cosmology, Variation of The Constants, Calculation Attempts
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