The full width at half maximum peak-height. The natural line width is the theoretical value of the full width at half maximum of the nuclear transition, usually calculated from lifetime data.
Source:
PAC, 1976, 45, 211. 'Nomenclature and Conventions for Reporting Mössbauer Spectroscopic Data' on page 213 (https://doi.org/10.1351/pac197645030211)