In the Bohr model, the Bohr radius $a_0$ is the radius of the lowest-energy electron orbit ($n=1$). From a quantum mechanical perspective, the radial probability density $P(r)=4\pi r^2|\psi {|}^2$ for the 1s state of hydrogen is maximum at $r=a_0$. Therefore, $a_0$ is the most probable distance of the electron from the nucleus in the ground state.