= Physical constant
A <physical constant> is a quantity whose value is independent of the particular <experiment> within a stated <physical theory> and regime. Examples include the <speed of light> in vacuum and <Planck constant>. A numerical value depends on the <units of measurement>; a <dimensionless constant> avoids that dependence.
Some constants have exact numerical values by the definition of the <International System of Units>. Others, such as the <fine-structure constant>, remain measured quantities with experimental uncertainty. Exact unit-defining values do not imply that every realization of those units is exact.<The International System of Units (SI), ninth edition, SI Brochure>[§2.2, defining constants and unit definitions]{ref}
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