The International Celestial Reference System (ICRS) is a coordinate system used for directions in the Celestial Sphere, fixed to the stars/constellations as would be seen from the Sun, or more precisely, from the Barycenter of the Solar System. It specifies the coordinate's Epoch as J2000.0. It was developed by and is considered standard by the International Astronomical Union.
ICRS can be seen as a precise standardization of the Equatorial Coordinate System, using a Frame Of Reference representing the calculated position of the barycenter at the specified epoch based upon the positions of more than 100,000 astronomical objects to provide a very high precision, which is of use in the age of Very-long-baseline Interferometry. The angular differences between ICRS and J2000.0 equatorial-system coordinates amount to only few milliarcseconds.