isomer shift

in Mössbauer spectroscopy
Measure of the energy difference between the source (E s) and the absorber (Ea) transition. The measured Doppler velocity shift, δ, is related to the energy difference by \[E_{\text{a}}- E_{\text{s}}=\frac{\delta \ E_{\gamma }}{c}\] where E γ is the Mössbauer gamma energy and c is the speed of light in vacuum.
PAC, 1976, 45, 211. 'Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data' on page 214 (