A Physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time.
It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.
There are many physical constants in science, some of the most widely recognized being:
the speed of light in vacuum c, Newton's gravitational constant G, Planck's constant h,
the electric permittivity of free space ε0, and the elementary charge e.
Physical constants can take many dimensional forms, or may be dimensionless depending on the system of quantities and units used.
