The Rydberg constant is a constant that appears in the Balmer/Rydberg formula, which describes lines in the spectrum of radiation emitted by a hydrogen atom.
The Balmer/Rydberg formula gives the wavelength of the radiation emitted by transitions between energy levels in the hydrogen atom. The inverse of this wavelength is equal to R(1/n1^2 - 1/n2^2), where R is the Rydberg constant, and n1 and n2 are the numbers of the energy levels involved in the transition. The Rydberg constant is approximately equal to 1.10 x 10^7 inverse meters. It is named after Johannes Rydberg, who derived the formula for energy transitions in the hydrogen atom.