High Quality Content by WIKIPEDIA articles! In physics, a dimensionless physical constant (sometimes fundamental physical constant) is a universal physical constant. Because it is a dimensionless quantity, its numerical value is the same under all possible systems of units. Fundamental physical constant may also refer (as in NIST) to universal but dimensional physical constants such as the speed of light c, vacuum permittivity 0, Planck's constant , or the gravitational constant G.