The bibcode (also known as the "refcode") is a compact identifier used by a number of astronomical data systems to uniquely specify literature references. The Bibliographic Reference Code (REFCODE) was originally developed to be used in SIMBAD and the NASA/IPAC Extragalactic Database (NED), but it became a de facto standard and is now used more widely, for example, by the NASA Astrophysics Data System who coined and prefer the term "bibcode". The code has a fixed length of 19 characters and has the form