free core nutation (FCN)

free mode in the motion of the Earth's rotation axis with respect to the Earth, due to non-alignment of the rotation axis of the inner core with respect to the mantle; the period is retrograde diurnal in the terrestrial frame and prograde long-period in the celestial frame.

Source: IERS Technical Note 36: Glossary (PDF, 244KB, Not barrier-free file.) of the IERS Conventions (2010)