Bohr radius is fundamental physical constant

$$\displaystyle a_0={\hbar\over\alpha \mu c} = \frac{\hbar^2}{\mu e^2} \approx 5.2917721067(12) \times 10^{-11} \rm m$$

where
$$\hbar$$ is the Planck constant, $$\hbar \approx 1.054571800(13)\times 10^{-34}\, \rm Joule\, second$$
$$e$$ is elementary charge,
$$\mu$$ is electron mass, $$\mu = 9.10938356(11)\times 10^{−31}\, \rm kg$$;

$$\alpha = \frac{e^2}{\hbar c}$$ is thin structure constant,
$$c$$ is speed of light,

The Bohr radius appear as scale of the solution of the Radial equation for hydrogen atom‎; roughly, it determines the size of all atoms to be of order of several picometers.

Usually, letters $$a$$ and $$c$$ are used to denote coefficients of some polynomial and/or asymptotic expansions. In this case, it worth ro denote the Bohr radius with $$r_{\rm b}$$ or $$r_{\rm B}$$ or $$R_\rm B$$