A small difference between the different isotopes of an element in the transition energies corresponding to a given spectral-line transition. For an electronic spectral-line transition between two energy levels a and b in an atom or ion with atomic number Z, the small difference ΔE_ab = E_ab(A′) − E_ab(A) in the transition energy between isotopes with mass numbers A′ and A is the isotope shift. It consists largely of the sum of two contributions, the mass shift (MS) and the field shift (FS), also called the volume shift. The mass shift is customarily divided into a normal mass shift (NMS) and a specific mass shift (SMS); each is proportional to the factor (A′ − A)/A′A. The normal mass shift is a reduced mass correction that is easily calculated for all transitions. The specific mass shift is produced by the correlated motion of different pairs of atomic electrons and is thus absent in one-electron systems.