isomer shift

in Mössbauer spectroscopy
https://doi.org/10.1351/goldbook.I03296
Measure of the energy difference between the source (Es) and the
absorber
(Ea) transition. The measured Doppler velocity shift, δ, is related to the energy difference by EaEs=δ Eγc where Eγ 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 [Terms] [Paper]