An atom in which one of the electrons is in a highly excited state and therefore has a very high principal quantum number. Physically, this means that, on average, the electron is much further away from the nucleus than an electron in its ground state. This in turn means that a Rydberg atom is much more responsive to an electric or magnetic field than a normal atom. Rydberg atoms occur in astrophysics and in plasmas. They can be analysed theoretically using either the semiclassical approximation or the Bohr theory and its generalization to elliptical orbits.