Epoch (J2000)
Also called: J2000, precession.
The date that coordinates refer to — usually J2000, the start of the year 2000 — because the sky's grid slowly shifts.
The Earth's axis wobbles like a spinning top over 26,000 years (precession), so the grid of right ascension and declination slides slowly against the stars. Catalogs give coordinates for a standard epoch, J2000; for the next few decades the difference is a small fraction of a degree.