The bibcode (also known as the "refcode") is an 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
where YYYY is the four-digit year of the reference and JJJJJ is a code indicating where the reference was published. In the case of a journal reference, VVVV is the volume number, M indicates the section of the journal where the reference was published (e.g., L for a letters section), PPPP gives the starting page number, and A is the first letter of the last name of the first author. Periods (.) are used to fill unused fields and to pad fields out to their fixed length if too short; padding is done on the right for the publication code and on the left for the volume number and page number. Page numbers greater than 9999 are continued in the M column. The 6-digit article ID numbers (in lieu of page numbers) used by the Physical Review publications since the late 1990s are treated as follows: The first two digits of the article ID, corresponding to the issue number, are converted to a lower-case letter (01 = a etc.) and inserted into column M. The remaining four digits are used in the page field. Some examples of the code are as follows:
|1974AJ.....79..819H||Heintz, W. D. (1974). "Astrometric study of four visual binaries". The Astronomical Journal 79: 819–825. Bibcode:1974AJ.....79..819H. doi:10.1086/111614.|
|1924MNRAS..84..308E||Eddington, A. S. (1924). "On the relation between the masses and luminosities of the stars". Monthly Notices of the Royal Astronomical Society 84: 308–332. Bibcode:1924MNRAS..84..308E.|
|1970ApJ...161L..77K||Kemp, J. C.; Swedlund, J. B.; Landstreet, J. D.; Angel, J. R. P. (1970). "Discovery of circularly polarized light from a white dwarf". The Astrophysical Journal Letters 161: L77–L79. Bibcode:1970ApJ...161L..77K. doi:10.1086/180574.|
|2004PhRvL..93o0801M||Mukherjee, M.; Kellerbauer, A.; Beck, D. et al. (2004). "The Mass of 22Mg". Physical Review Letters 93 (15): 150801. Bibcode:2004PhRvL..93o0801M. doi:10.1103/PhysRevLett.93.150801.|
- M. Schmitz; G. Helou; P. Dubois; C. LaGue; B.F. Madore; H. G. Corwin Jr. & S. Lesteven (1995). "NED and SIMBAD Conventions for Bibliographic Reference Coding". In Daniel Egret & Miguel A. Albrecht. Information & On-Line Data in Astronomy. Kluwer Academic Publishers. ISBN 0-7923-3659-3. Archived from the original on 7 June 2011. Retrieved 2011-06-22.
- "The ADS Data, help page". NASA ADS. Archived from the original on 14 October 2007. Retrieved November 5, 2007.