= Dimensionless constant
A <dimensionless constant> has no residual physical dimensions, so changing units leaves its numerical value unchanged. The <fine-structure constant> $\alpha=e^2/(4\pi\epsilon_0\hbar c)$ is an example: the dimensions carried by its constituent quantities cancel.
A dimensional constant such as the speed of <light> can be assigned the numerical value one through a unit convention; this cannot make an arbitrary dimensionless coupling equal one. Dimensionless ratios also distinguish physical regimes, but not every such ratio is a universal physical constant. A <Reynolds number>, for example, depends on the selected flow, length and material properties.
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