nuclear quadrupole moment (spectroscopic)

A parameter which describes the effective shape of the equivalent ellipsoid of the nuclear charge distribution, Q > 0 for prolate (e.g. 57Fe, 197Au); Q < 0 for oblate (e.g. 119Sn, 129I) nuclei.
Source:
PAC, 1976, 45, 211 (Nomenclature and Conventions for Reporting Mossbauer Spectroscopic Data) on page 214