Definition: A Rydberg state occurs when an atom or molecule becomes excited such that one of the
electrons moving into a high principal quantum number orbital. Such a "Rydberg atom" is extremely sensitive to the influences of external fields, collisions, reactivity, and microwave radiation. As such, atoms in the Rydberg states are of interest in a wide range of modern experiments.
This excited state is named after Johannes Rydberg, who devised the Rydberg formula in 1888. This formula depicts the change in energy of an excited atom and, therefore, was useful in determining the energy of a photon released during such excitation.
In the Rydberg formula, the value n represents the principal quantum number.