Short for "CATalog of the New York Public library". Apart from being what is quite possibly the cutest acronym ever, CATNYP is the NYPL's online and in-house catalog of all 3,000,000-plus volumes belonging to the research libraries that have been catalogued since 1972, as well as the more than 2,000,000-plus records for roman alphabet books and periodicals catalogued prior to 1972.
The books listed in CATNYP do not circulate.
Apart from being used by library staff and the public to check on volume availability, select staff are granted hardcore, 'are-you-sure-you-want-to-delete-every-record-in-the-database?' -type access that, among other things, gives you the ability to search by barcode as well as the ability to edit item and bibliographic records for the purpose of record clean-up and preservation. Maintaining a database that size (as well as the books the database refers to) is an ongoing project.
CATNYP's public web-based incarnation is available at http://catnyp.nypl.org/