Physical constant

A physical constant is a physical quantity that is believed to be unchanging over time, either because it represents a fundamental property of the universe (e.g. the electron mass) or because of its relation to a given system of units (e.g. the Avogadro constant). The number of possible physical constants is infinite, although some are considered much more important (or "fundamental") than others.

The value of a physical constant must usually be determined by measurement, although some constants are used as bases for the International System of Units (e.g. the speed of light, the magnetic constant) and so have defined values in SI units: nevertheless, even constants with defined values in SI units could in principle be measured in a different system of units. This distinguishes physical constants from mathematical constants such as π and e, which are defined by mathematical expressions.