Hipparcos was a space observatory operated by European Space Agency from 1989 to 1993 to perform astrometric Surveys.
Results were the Hipparcos Catalog, and through further analysis and comparison, the Tycho Catalog (or Tycho-1 Catalog), and based on it, the ACT Reference Catalog (for Astrographic Catalog / Tycho, aka ACT or ACT-RC), the Tycho Reference Catalog (TRC). and the Tycho-2 Catalog. All the latter are generally referenced/cited by the "TYC" designators.
All these are concerned with the exact positions of many stars at any given time, useful as guide stars, e.g., for very accurate determination of where a telescope is pointing.