The Bohr radius is a fundamental physical constant that represents the average distance between the nucleus and the electron in a hydrogen atom in its ground state. It is approximately 0.529 angstroms, or 5.29 x 10^-11 meters. This concept is derived from the early model of the atom proposed by Niels Bohr, which describes how electrons orbit the nucleus in quantized energy levels. The Bohr radius is significant in quantum mechanics and atomic physics, as it helps define the size of hydrogen-like atoms. It also plays a crucial role in understanding atomic structure, electron behavior, and the interactions between atoms in various materials.