One of three quantities, known as transition probabilities, used to describe the rate at which photons are absorbed or emitted from atoms or ions. The Einstein coefficients are widely used in the understanding of stellar spectra. For a bound–bound transition, i.e. a transition between two bound states 1 (the lower state) and 2 (the upper state), the Einstein coefficient for absorption is B₁₂ and for spontaneous emission is A₂₁. The radiation field causes stimulated (or induced) emission, with the coefficient B₂₁.