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.
Source:
PAC, 1976, 45, 211. 'Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data' on page 214 (https://doi.org/10.1351/pac197645030211)