epoch

a fixed date used to reckon time for expressing time varying quantities. It is often expressed in the system of Julian date, marked by the prefix J (e.g. J2000.0), with the Julian year of 365.25 days as unit. The term is also used to designate the date and time of an observation, e.g. “epoch of observation”, which would be better expressed by ‘date of observation’.

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

Original Source: IAU 2006 NFA Glossary