International Celestial Reference System

The International Celestial Reference System (ICRS) is a celestial coordinate system fixed to the stars/constellations as would be seen from the Sun, or more precisely, from the barycenter of the solar system. It was developed by and is considered standard by the International Astronomical Union (IAU). ICRS can be seen as a precise standardization of the equatorial coordinate system using a frame of reference that is presumed to be stable for practical purposes, centered on the solar system's barycenter at the specified epoch relating the barycenter to measurements of distant objects. ICRS presumes the coordinate's epoch as J2000.0. The angular differences between ICRS and (non-ICRS) J2000.0 equatorial-system coordinates amount to only few milliarcseconds which is significant in the age of very-long-baseline interferometry (VLBI).

ICRS merely specifies this overall goal, and for determining an object's coordinates, it references a specific appropriate celestial reference frame that can be reproduced, from observation of astronomical objects that are virtually stationary. The reference frame is subject to future improvements or replacement, with upgrades for accuracy and precision as new data and analysis make this possible. Currently, a version of the ICRF (International Celestial Reference Frame), ICRF3 is used, which is based upon a set of extragalactic radio sources such as quasars that, for practical purposes, are stable, using VLBI for precise measurement. For optical, a version of the Gaia Celestial Reference Frame (currently Gaia-CRF3, the Gaia Celestial Reference Frame 3) is used, which is based upon the positions of a set of more than 1.6 million astronomical objects. A Gaia-based reference frame was first adopted in August 2021, replacing the previously-used Hipparcos Celestial Reference Frame (HCRF).

Note: I've seen the names cited (far less often) as the International Coordinate Reference System and International Coordinate Reference Frame, clearly for the same system and frame, but I find no confirmation that these were ever the official names.