equation of the origins (EO)

distance between the CIO and the equinox along the intermediate equator; it is the CIO right ascension of the equinox; alternatively the difference between the Earth Rotation Angle and Greenwich apparent sidereal time (ERA − GAST).

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

Original Source: IAU 2006 NFA Glossary